You have, what is known as, a "Stock Photo" or "Photos". These people don't want the print, they want the digital image for use as advertisment to generate interest in what they have to sell (spectators to their events). They liked your image(s) and would like to use them in a campaign to get more butts on seats. This is what you have. Stock Imagery.
There are basically 2 types of stock. Advertising and Editorial. Advertising may use an image to promote (multiple uses). Editorial may use an image of the event to highlight or run a story (generally 1 time use unless license is issued for newsagency stock reasons).
Either way, both types of stock should be accompanied by a license. You have the information you need from them:
They plan on printing 400 t-shirts @ $20/shirt.
You should produce a license for this application. Here's where you buy into the negotiation. If they supplied t-shirt's for sale with nothing on them, how many do you think they'd sell? None right. Now, if they had "X Car Club" on them, how many do you think they'd sell? Educated guess would tell me probably less than 50/400. Their expectancy is to sell at least 75% of those t-shirts and unless there is something i'm not seeing in your post, looks like your image has alot to do with getting those sales figures. You have to come up with a percentage of "potential sales". I doubt they are producing 400 shirts to hopefully sell 72 of them. They intend on selling 400. Do not be swayed by this whole " breaking even line". A licensed image is a licensed image and should be priced accordingly. You have to assume the potential is there for them to sell 400 shirts. Here's the math as a guide:
Research first -
A 100% cotton, preshrunk, blank tshirt is about $3-4 landed.
Silk screening per shirt on a 400 quantity will be a generous $8 per shirt.
Total cost to Car Club for 400 shirts - approximately $12
Selling for $20 is a profit margin of 40%. Break even my ass!
If they sell 400 shirts (it's not your fault if they don't), here's the math for sales:
375 (25 less for giveways) shirts at $20 ea = $7,500. Take 40% margin off that for their potential profit and that leaves an actual cost to the club of around $4,500 (you have yet to get a slice of this action, after all, your image helped sell that many). This is where you have to be comfotable in what you did versus what you want. As I said earlier, 5-10% of sales is very fair considering your image is doing alot of work. Here we go with the math again:
$8,000 in potential sales (you don't count the 25 giveways to Granda Jo and family!)
A very conservative 7.5% on your behalf = $600
Let's go back to their cost figures again. Remeber that thier cost was an estimated $4,500 before you stuck out your hand; less $600 and they are looking at a cost of $5,100. If they sell 375 shirts at $20 each, they stand to make $2,400 profit, that's a profit margin of 32%. Any business person on here will tell you that's a great margin to work at.
The research I did was straight off the internet. I am a photographer who has a business. I am not a slik screener nor am I a shirt retailer, but the information is staring you right in the face. If you are getting into the world that your images has a price tag, make the price tag right, don't think your image is too much, know what you need to charge for a fair price. Are you really going to give up an image for $120 for a tshirt that could generate them not only potentially $2,400 in profit but potential revenue at another event because there is 400 tshirts on people's backs circulating around the area?
$600, for the license to use a selected image for a number of times not exceeding 400 for the purpose of t shirts used as an advertising medium. Should be stated in a basic license to this club.
If it were me, and they baulked on this, I would go back to being a spectator and taking happy snaps from the stands. This whole market is heading down the crapper because people expect images for nothing. No one said you can't be flexible and offer new client discount or similar, but if they have you for $120 on this go around and they actually ask you to be their "track photographer", you will be offering 8 x 10's for $6.50! before you know it.
Posters. You never stated if they are selling them or not. Really, it doesn't matter. If they are being circulated amongst the community, you need a new license with the similar math above. It is still a form of advertisement which has the potential to generate ticket sales for future events.
At the end of the day, you need to do what you think is fair for you. You also need to be very, very conscious that the event in 2010 has the potential for 2500 people at maybe $12 ea. That's $30,000. You think $360 for both the used mediums to help attain those figures are justifiable? Your not being greedy. If they dont go for it, even if you can justify your invoice. they are really not worth working with, because they will nickel and dime you to death.
License, license, license.