For over two years now I have had pfSense (full install) running off a Transcend 1GB DOM (Disk On Module) which is flash memory (completely quiet, no heat I can detect with my fingers) on a IDE connector to plug into the motherboard IDE socket. Its been designed to act as a disk replacement (so may well give better life than flash drives for applications that do only occasional writes). From memory, the cost was a bit over 10 euros. My system has enough memory to not swap.
From what other people have written in the forums I expect you would get reasonable life out a Compact Flash card (and IDE adapter) running the full version of pfSense provided you didn't swap a lot or run packages that do a lot of disk writes (e.g. web cache).
For the price of flash drives currently I suspect you could easily get a couple of 4GB drives, install on one, use it as your boot drive and take a physical copy to the other drive which you could put aside as a spare in the case the install drive dies. If you don't do too many writes to the drive you could well get a good length of service from it. And it it dies, you can instantly replace it with your spare.