I'm not sure it would be worth the time vs the money. Most of my conversion/ repaints took less than 30 minutes. I wouldn't feel comfortable charging more than 10$. At that point it wouldn't be worth my time
As winterdyne said above, dont undervalue your time, minimum, I would say, I'd double the cost of the mini for a repaint, adding magnetes will drive pricing up. It really depends on the flexibility of the market, I have seen WWI plane repaints for wings of glory go for over $100 on ebay, and yes that is a specific market that depends on what people will pay, and it is a vicious market, but when figuring pricing, remember, your time is valuable.
I figure, I'm a half decent painter and am experienced at some aspects of remodding some of these.. 30 bucks a mini, if you send me your small based mini, I think would be where I wouod start, if I was inclined to take on commissions. And probably 50 for large base ones. That would just be painting.. no mods or anything.
Also, make sure you give a reasonable time frame for turn around, know your limits and make sure you dont have an angry customer emailing you every other day. Make sure you have the time to begin with.. some one single, might have more free time than a married guy that has a honey-do list for the weekend, or evenings.. even though it brings in money, the wife is smart enough to know the money will probably find its way back into the hobby, either in paint supplies or new toys for daddy.. lol a lot could depend on her generous nature, or her own love of all things geeky.. some guys are lucky, others.... just have incredible wives with no geek gene.. haha
Just how I see this situation. Also a good dialogue with the customer to get an idea of what is expected, you dont want them to be asking what the heck you sent them when you're done.. and also be wary of .. "oh, just use your imagination, anything works" issue.. lol.. anything doesn't always work, and sometimes what you think is cool is not how others see it..
"I mean, come on kid, my armor's older than you are.."
"Hahaha. Very funny, at least I only hit the enemy target, Ya collateral bastard."
Corp. Haig Talin, MK - 472
Armor 18 - Fire - N - Ice 181st Mechanized Armor Assault Div