that filing fee would be for a provisional application. this covers your invention for one year from the date of filing, at that point you MUST file a regular utility application, but you get the benefit of the earlier filing date for any protection.
this is good if you try and market your invention to other companies for sale.
you MUST NOT publically disclose or offer for sale your invention before you file, not without a non-disclosure agreement. if your invention would be worth of foreign protection, you've got 1 year from filing, unless you've already publically disclosed the info, then from what i recall, you're screwed.
i am a paralegal for an Intellectual Property Law firm, and usually we have summer associates or just low level associates do work for smaller clients to save costs. either way, you're looking at spending over US $1,000 to get a patent, more likely 2-3gs for it. if it's in any way technical, you're going to be hard pressed to get the protection you want out of your invention without an attorney...
keep doing your research, and keep it to yourself. if you want any help or anything, send me a PM and i'll see what i can do.