Invite is a flat fee cause you get freemium, open and im you have to pay like 7 bucks a month (or 20ish for 3 months which is basically the whole season) plus the league fee which I believe is 10 and 15 bucks respectively
first place in open and im is $166.66 and $250 per person (respectively)
not a bad deal if you're winning, harder to justify spending $85 to lose to banny though if you aren't making playoffs (for which the minimum 4th place gets like $400 per person iirc)
edit: might be a good idea to include the roster & past experience or something so people have an idea who you are and why they should give you free money (dunno if you have a finalized 6 yet and i suppose it probably depends on if you get a sponsor and play invite because certain people may not be able to/want to play)