An ssd will de facto be much faster than a HDD - especially under normal use - since the seek times are not touched by bandwidth (which is mainly the sata2 interface's "problem") - Everyday use involves a lot of small-file access, with lots of seeks, rather than big files, where the transfer rate becomes very important.
I'd still stick to the M4 drive, they are hovering around the 50c/GB mark IIRC and provide excellent performance and reliability for the money.
Samsung are even better it seems, however they're more pricey.
If you're willing to take an anything goes approach, then of course you can find even cheaper stuff, like sandisk. (their memory cards are good, so in theory they should be all right, I haven't tested them and haven't seen any reviews either that I'd trust)
Eg sandisk 240GB for ~$100 https://www.ozbargain.com.au/node/202819
If you want the cheapest yet still good, you probably need to do your own research.
p.s. pccg is a good store indeed! And they have the sanDisk drive for $115.