There's a guy, Gavin, who has brought cars specifically "for rent" to Thunderhill. They are well sorted, well prepped Lemons veteran cars. He offered a pricing structure that varied from "take it as-is, as it came off the track last race including bald tires and all" to "I'll do everything for you including new tires, all refueling pit stops and car maintenance during the event." Fish did/does that with the Model TGT and with his brown Pintos and I've seen him transport those cars all the way to Houston from CA for a team who rented them.
I rented out my Hyundai to a team in Inde who had their car crap out early. I based what I charged them on what it costs to field the car normally, which is about $2500 for the weekend. Minus $1380 entry fee, it runs about $1100 just in what the car actually consumes through an uneventful weekend, no breakages no accidents, and there's still enough tire and brake at the end to run at least another day but that also includes transport to and from the track. That's based on about 4gph on track fuel consumption. For easy math, call that about $85/hr just to break even on the car owner's effort and the car is rented for the entire 13.5 or so hrs. If someone only needs it for a day and the car sits in the paddock for a day not recouping $, figure the single day cost should be $170/hr minimum just to break even, and so on.
Edit: oh, and after being a habitual Late-Tech-er, I found it actually mutually beneficial to run the spare car through tech on Friday during a lull and when they are already set up for tech with people qualified to do the safety inspection sitting around waiting for tech to close. Come Saturday or Sunday, those people aren't always available to inspect a car. ***Now I'm NOT saying this is the correct or approved way of doing it***, but what I did was have the car safety inspected and signed off and then I HELD ON TO the signed inspection form. If the car were to be put into action, that form AND CAR is then brought to the penalty box with the correct number and transponder already on it to show the judges "THIS CAR is now going into action to replace XYZ. Please inform the tower of the year, make and model (and color change, if necessary)" .Then the already completed form is walked over to HQ and turned in and then I also ask for a new sticker that is then placed on the backup car before it heads out on track. I write very clearly "BackUp Car" on the tech form and make it clear that the original car absolutely will not go back out on track and that the switch is a permanent one for the rest of the weekend, and we forfeit any consideration for any awards or trophies It feels like the teams who stick with their original bad plan are the ones who should be rewarded for their futility. YMMV