Define "break the bank". All the seat bags I am familiar with are on the order of $100-$150, which is why I am making a seat bag based on JerryW's design
http://www.bikepacking.net/forum/index.php/topic,2611.0.html "Full DIY Bikepacking Kit". If interested in this route, check out the DIY forum; lots of useful info on it.
Alternately...you have anything on the handlebars? If not, you may be able to strap it on; a sleeping bag is a bit too big between the brake levers with my Woodchippers, but may work with a flat/riser bar. Alternately, you could go with a medium-light rear rack (if you are riding rigid or a hardtail) and strap your bag on there. Not as lightweight as a seat bag, but a cheap way to check out bikepacking setup without spending as much (or any, if you already have everything) cash.