I tune cars for money all the time (I have a crome dealer license). I charge $80/hr for street tuning. I start the clock the minute I open the hood of the car. I can't tell you how many people show up with check engine lights, missing or unplugged sensors, coolant leaks, broken or unhooked fan switches, oil leaks, wiring problems, etc. I never know what kind of crap they are going to bring to me. When I first started out I made business cards on some online website where you can design your own cards. I think I paid $60 for 1000 cards. I gave stacks of cards to my friends and they keep the cards in their cars. Whenever the see a "lowered" or "rice-rocket" honda parked they stick one of my cards under the windshield wiper. I also chip/repair/mod ecu's. I charge $60 to chip an obd1 ecu and I charge an extra $40 to convert a non-vtec ecu to a vtec ecu (I buy the ecu parts in bulk from digikey and mouser). I sell chipped/converted P06 ecu's for $160. I also offer package deals. For $250 I will supply an ecu and street tune any naturally aspirated car. For $300 I will supply an ecu and street tune and turbo car up to 12PSI. I will not street tune any car over 12PSI (it gets kinda scary driving on the street making any more power than that). For dyno time I rent a local dyno for $100/hr and charge the customer $175/hr. I also tune cars for local shops that supposedly "tune" cars. I give those guys a discount and they take the credit for the tune. In other words they pretend like they are the ones that tuned the car to make themselves look good. I also had stickers made that say something like "this ecu is for offroad use only"" that I stick over the center screw on the top and bottom of the ecu. That way I will be able to tell in the future if the guy messed around with the ecu after I chipped it.
As for motors blowing up and other problems that are bound to happen. The way I deal with this is I just tell everybody before I even touch their car that there is no warranty and that I am not going to take responsibility for anything that happens to there car during or after I tune it. If they have any problems with that policy or if I get the feeling they are going to bitch about problems in the future I tell them to f*ck off and I don't tune their car.
WATCH OUT FOR PEOPLE WHO KEEP NAMING A BUNCH OF DIFFERENT SHOPS/PEOPLE THAT WORKED ON THEIR CAR. THAT MEANS THEY ARE CHEAP ASSES AND THE ARE GOING TO BE A HEADACHE.
good luck and happy tuning
TOMDATA
P.S. A good thing to offer is a "valet" chip to the customer. Basically set the rev limiter at 5000 rpm and turn the vtec off. That way they can put that chip in the ecu if they are taking the car in for repairs or if they have to loan the car to someone else to drive. Its an easy way to squeeze an extra ten or fifteen bucks out of the guy after you are done tuning. I also upsell people on "race" and "street" chips. The "race" chip is usally a tune with a little bit higher revlimiter and closed loop disabled. The "street" chip will have a very conservative rev limiter and closed loop for better gas mileage. I also sell "kill switch" chips. These are usually sold with a moates two timer. I put the regular tune on one part of the chip and I put a "kill switch" on the other part. The "kill switch" simply has all of the fuel cells above 1200 rpm set to zero. That way the car shuts off as soon as it gets off idle. Just some ideas and examples of how to sell extra chips/hardware after you are done tuning the car.