Here is a non-product, business minded post.
The used car dealer is using your services to increase the value of the car (regardless if it is $50 or $500. There is no doubt about it. His point of view is: What can I do to the car to increase the P&E??. At this point he is not exactly thinking about you, right? So make him!
What are you offering that others won't:
1) Quality work
2) Reliability
3) Availability
4) Priority (you will offer to take his vehicles over others)
**which, from your OP, they had issues with from the prior detailer
You have to sell yourself, brother! I will be honest with you and tell you that the direction of your posts makes me cringe (as a customer more than a detailer). You are basically trying to undercut your own work to accommodate them. In the end, the only ones that lose will be the customer (getting subpar detailing job) and you, because ultimately your name will be associated with those "polished turds". Someone will find out who details those guys car, then your name will be tagged to any disgruntled customers looking for a reason to criticize. They may have nothing against the finish on the car and may only have issues with the mechanicals of the car, but I guarantee you that you will thrown in like the rest of them.
Don't get me wrong, I am not saying your work is subpar (Saturn looks pretty dang good), but it is quite obvious from the way you originally treated it (without the pressure of staying on an unrealistic budget) that nothing less than what you did to the Saturn would have been your first choice.
Again, my approach would not be to be on the forum asking for advise on how to cut more corners, but would concentrate on selling your $100 detailing job.
If they are any kind of businessman, they will understand numbers. You can always present a breakdown of what it costs you to do a job (overheads, products etc...) but don't sell yourself short. You need to include the rent valuation in your overheads even though you are not renting from yourself. Reason being is if you are actionable in a liability case, they will go after your insurance, your "business" insurance (home) and any assets they can go after.
Side note: I am pretty sure that you are writing off on your income tax a portion of the sq.ft. of your garage for your business, aren't you? (if not that is pretty silly.... unless you are strictly under the table) So even though you are not "charging" yourself rent, you have to figure your cost "in case #### happens", because based on your taxes, your garage is associated with your business and owner/operator become liable.
Ultimately they will understand the numbers, but best is to sell your attributes (points listed above). It is always worth more to me to have a loyal, dedicated, available sub then to have a cheap one

rops:
A few more thoughts to put in the pot.
1) You definitely need a sit down with them to see what exactly they want and see if you are willing to accept the level of work and the fact that your name will be associated with those car
2) You can approach a potential discount by explaining that you discounted your $125 to $100 in an attempt to get more business from them and will be glad to drop the prices to $90-$95 (for cars... not SUV or large trucks) if they allow you to put your business card in the glove box of each car you take care of. You make it cheaper for them and you get legit retailer advertisment Im the MAN