You sound a lot like me, although I live in southern california and plan to snowboard a little more often than you.
I recently got 2 board/binding setups off craigslist -- one for me, one for my girlfriend. The craigslist ads were actually posted for pawn shops.
Got a limited transition and a lamar cruiser for 100 and 125 respectively. They are practicially in new condition and were probably only ridden a few times each. Got some inexpensive boots at Big5 after that for $50 a piece.
We wanted to get some boards as opposed to renting too. Our logic was like yours -- renting gear can get expensive fast. After about 3-4 rentals you end up saving money, plus you can get used to your own board. I did the same thing with golf when I first started playing...not sure if its the same idea with snowboarding...but I'd prefer to use my same less than average clubs than rent a set each time out because even though the clubs aren't the best, you get a feel for them.