I don't really think SSDs speed up things at much as people claim. Assuming you have enough RAM, after the first run its still right there in RAM.
I would just look at the capacity you need and go from there.
Online is a better bet than a big box store, but then again I have seen some Wal-marts still selling a 1 TB portable drive for $90.
Technically, you could rip the case apart and have a 2.5 in SATA drive all set to plug in somewhere.
If you need consistantly better read/write performance regularly, then that's when I'd consider an SSD. It's easier to put these in before installing Windows.. vs trying to move things around later