Ima have to agree with Cawwa (and anyone else who's said it), this isnt the league to be doing this with, for numerous reasons.
1) I hate the premise of pay to drive, i know this is 'donate if you want', but your already talking about it as a possibility and even if you werent its still 1 step closer to that, and pushing the repeated "we'd really appreciate some money" line is pretty close to requesting the money, especially if your even going to set up a pricing structure, enforced or otherwise.
2) Its a league which i was under the impression was set up kinda like a feeder series for people to gain experience, limiting it to the lower ranking drivers like myself who arent quick enough to drive in MoE/IGTC etc but still have a desire to race in such a league for fun. Throwing in the competitive element of 'and the prize money is £xxx' makes this far from Amateur, and theres more of an incentive to win races with the best drivers you'll allow than there is of letting inexperienced drivers take the wheel. If i hand my username over to bawbag, and our other n00bs do the same, its easy money.
If you do this with NDR Uber Kick-Ass League, anyone can enter and win some fancy shiz, then thats great, its fair and transparent. If you do it with restrictions on who can enter and who cant, based on the fact that they're too GOOD to compete, thats wrong, and it doesnt take a genious to see how it can be abused.
Theres plenty of projects in LFS where a simple donation option is available, and leave it to peoples generosity to assist paying for some of the unavoidable expenses, but structuring fees for a season and rounds, for teams and driver... thats stepping out of the boundary for generosity and putting emphasis on whats an acceptable donation.
Donation button - Thats perfectly reasonable
Asking for donations - Reasonable, though not that different to standing in the street with a collection bucket and jigging it as people walk past, which isnt allowed these days.
Deciding how much people should donate - Rebrand to NDR Corp.
Pricing wise, its reasonable, i cant argue its too much to ask for, but the manner in which i feel your going about it all i dont really like, no matter how bold the big red text is, personally i grew up not asking for things, maybe as a kid there was plenty of 'can i have some sweets' but i knew from a reasonable age that if i deserved something and my folks were able to provide it, then if i was lucky then i might get that. Whether it was a bike for xmas, or a small treat as daft as an ice cream.
So from my upbringing asking for something isnt really in my nature unless i have little option but to ask (I have a crapton of debt, earn an absolute pitance of 3rd world standard, and i pay that months debt off and move onto the next month without spending anything on myself, yet my folks could clear it without batting an eyelid). Im not saying its wrong to ask for help, but in this way, i find its too emphasised on guilt and pressure than honest generosity. To be clear, im not critisising anyones upbringing either, just explaining my feelings and why.
Also, drifting OT a little, all the leagues which im aware of which involved fees and/or prizes have an amazing knack of disappearing mid-season too. eRacing or whatever it was called vanished, their big selling point being you could race in the top tier league with the top drivers, and in there they have decent prizes (ofc realisticly only ~5 people had a chance!) and those drivers didnt have to pay a penny to stand a chance of winning something, and the slower drivers were the ones expected to fund it all!
oneGP (09) had plans to charge teams to enter their blatent rip-off league (description text, rules, get rich quick plan...) which the original (FSR) charges high 3-figure sums per team, and OneGP claimed to offer 4-figure prizes and announce sponsors, went strangely silent on sponsors and prizes, and then prompty disappeared after about 4-5 rounds.
It seems anything with the promise of goodies turns sour, unless the dev team are running the show.
Regardless of all that, i'd strongly advise against using any proceeds on prize money for GTAL, its your league to run how you think best, but IMO its completely the wrong league to be incentivising results and removes what makes this series different from the likes of MoE/IGTC, the opportunity for regular racers to compete without looking like complete n00bs from the top drivers, we dont need their help for that