He seems determined to become an inventor and if it's fun for him, I don't really see the harm. Money sinks are good, so I put together these rules. This is all still a work in progress, of course.
INVENTION RULES
There are two kinds of inventors:
Tinkers. Natural-born creative thinkers with a knack for figuring out how things work. These are Specialists/Rogues and their knack for invention is measured by their Tinker ability.
Sages. These researchers put in the time and effort to explore the science and magic behind the underlying concepts that allow for invention. Sages invent using their Intelligence modifier.
HOW TO INVENT
Step 1. Decide what you’d like to try to invent and determine Complexity
Step 2. Invest in the idea
Step 3. Spend time working on the idea and make weekly Progress checks
Step 4. Make Progress checks equal to the invention’s Complexity to finish the project
Step 1: To invent something, the player first needs to decide what his or her character will invent. The DM will apply a Complexity number to the idea based on the required level of knowledge necessary to produce such an item.
INVENTION COMPLEXITY
Complexity 1: Basic tool, no moving parts; minor knowledge of sciences required
No assistants required
Example: dustpan, surfboard, leather wallet
Example: dustpan, surfboard, leather wallet
Complexity 2: Limited moving parts or some knowledge of sciences required
One assistant required
Example: retractable baton; refillable pen; crossbow-fired grappling hook
Example: retractable baton; refillable pen; crossbow-fired grappling hook
Complexity 3: Some moving parts or much knowledge of sciences required
Two assistants required
Example: matchsticks; spring-fired dagger; water filtration; crock pot
Example: matchsticks; spring-fired dagger; water filtration; crock pot
Complexity 4: Many moving parts or some magic, rare knowledge of sciences
Three assistants required
Example: mechanical clock; glider; arquebus; plumbing; magnetic compass
Example: mechanical clock; glider; arquebus; plumbing; magnetic compass
Complexity 5: Great magic, countless moving parts, advanced science & tech level
Four assistants required
Example: steam-powered carriage; clockwork soldier; flying machine
Example: steam-powered carriage; clockwork soldier; flying machine
Step 2: An initial seed investment of (Complexity2 x d6) x 100 gold is required to begin work. This reflects the lion's share of the research and necessary materials for the project.
The character must also have access to a suitable workspace or laboratory. The work area must measure 10 square feet x Complexity. A 10x10 workspace is enough for the simplest projects (a desk and a quiet bedroom will do), while the most ambitious projects will require a 50’x50’ testing complex.
In most cities, an outfitted workspace will cost Complexity x 10 gold per week.
Assistants are NPCs or players with a minimum Tinker score of 2 or Intelligence modifier of +1. Hiring an NPC assistant will cost 100 gold per week.
Step 3: At the end of each week, the inventor may roll to see if they’ve made any Progress on their invention.
- Tinkers need to roll their Tinker ability or lower on d6.
- Sages need to roll their Intelligence modifier or lower on d6.
- A roll of 6 always fails.
If the roll was successful, Progress was made. Once the player makes a number of Progress checks equal to the Complexity number, the invention is complete.
If the check fails, the player must roll on the Invention Setbacks Table
INVENTION SETBACKS (d12)
2) Vandals have broken into your lab and smashed everything up. Your work has been destroyed and you’ll need to start from scratch, including seed money. The workspace is relatively unharmed.
3) Some of the research you’ve been basing your invention on is flawed. Lose any Progress you’ve made while working on the project.
4) Some lousy thieves got into your lab and made off with all your vital equipment. Work can’t proceed until you pay Complexity2 x d6 x 100g to replace the missing equipment.
5) Where the hell did it go? Someone made off with your notes. Lose one of your Progress successes.
6) Whoops, looks like you broke something important. Pay d6 x 100g to resume work on the project.
7) A rival inventor is convinced that you’re stealing his ideas. Roll a Charisma check or he’ll send thugs/the authorities after you. Until they’re dealt with, work can’t proceed.
8) What a waste of time! You need to clean out your lab or do some other time-consuming menial task that delays your project. It takes two weeks before the next Progress check can be made..
9) No major setbacks. Check again next week.
10) This task is too hard to complete alone. Hire another assistant for 100g next week to continue work.
11) Your latest experiment didn’t work out too well, but you’re on the right track. Receive a +1 modifier on next week’s Progress check.
12) A costly mistake was made, but a valuable lesson was learned in the process. If you pay Complexity x 100g, you can immediately retry your failed Progress check.
Step 4: Once the inventor has made Progress equal to the invention’s Complexity, he or she completes the project. The character gains (Complexity2 x d6) x 100 xp and gains a working prototype of the invention. It may be fragile and/or imperfect:
INVENTION QUALITY TABLE (d6)
1) Flawed prototype. Will work once before failing miserably.2) Poor-quality prototype. The invention will work d6 times before failing.
3) Shabbily-made prototype. Poorly constructed, the invention is more fragile than intended.
4) Standard prototype. All features are working as intended.
5) High-quality prototype. All features work well and the device is sturdier than normal.
6) Superb prototype. All features work as intended and the device is perfectly constructed. Duplication checks are +1 when using this prototype as a model.
The DM should then set a price for the materials required to duplicate the invention. Creating a duplicate requires paying this cost and succeeding at an Invention (Tinker or Int mod) check. The Invention Quality Table should be rolled for each duplicate. A sale price for the invention can be set by the player, but what the market is willing to bear is entirely up to the DM.




