I would recommend set the event in a city where there is a convention going on. Those who wish to meet at a park/campsite location can still do so in the same city where the convention is happening and probably make arangements via email or phone to those in charge of that location to have this set up. Or you have the option of picking a locale/park that doesnt' require making such arangements (ie. parks or places that you don't need to pay for rent, just grab a few tables and enjoy till the security kicks you out).
Meanwhile, those who have the money and wish to experience the convention -can- and also enjoy the ISRP meet. Hopefully this will bring in a larger number of people.
The best of two worlds. For those dressing up, a Convention or a Ren Fair would be very good optional choices.
For those who don't have the cash, i would suggest saving a dollar a day, start today. Don't spend that dollar, keep it in a nice and hidey place where you won't touch it or be tempted to spend it. By the time the convention comes around (probably next Summer) which also happens to be convention and Ren Fair season, you should save 350 dollars roughly. Add an extra dollar to that dollar..double the results, add a third dollar to it, and triple the results, etc. See a Travel agent, this will reduce your travel fare by alot. Those who have the time and gas, road trip it!
For spending your good hard earn cash, the ISRP should be one fun event that be worth your payday and vacation, because this is a vacation not just a meet.