A patent will cost you at least a five figure amount in the first ten years.
the patent itself, a lawyer (and you want one), investigation fee by the patent examiner (that amount drops if your lawyer knows what he's doing)
and renewal fees
And worse...
every commercially at least somewhat successfull or promising patent will end up in court at one point,
since others can still try to have it removed again. (lack of innovation, similarity to something they did already or something they can 'proove' to have sectretly started to develop before the patent was granted)
And that's where you'll have to spend most of the money.
they only need to keep the case open for long enough and you'll run out of money.
So be prepared.
DO NOT just mail anything to yourself.. that's a movie myth, try to find any example online of people that successfully have proven their innovation and date like that.
you can of course.. but that's not going to count for much.
(we all know how to get in and out of a sealed envelope... heck I could even fake a stamp date if I wanted)
Instead ask a civil law notary to date and sign your paper (best have him store a copy)
that shouldn't be too expensive and in court that date will be bulletproof.
So if all you're after is a date... notarially certified date on your paperwork is the way to go.
'sid