I've been looking into the exact same thing. I have found that a Vivitar 283 flash head mounted on a cheap tripod (it has a tripod head in the base) or an inexpensive lightstand and an umbrella should be pretty reasonably priced. Then I'd suggest you get a Wein flash slave that plugs directly into the Vivitar flash head. It's triggered when your on camera flash fires. The Vivitar 283 can be had on Ebay for ~$25.00. Lightstands can be found at http://bhphotovideo.com for ~$35.00, as well as the Wein slave for ~$20.00 and an umbrella for ~$20.00 So for around $100 you get a off camera flash that's diffused and works well. Of course, like Dave said, you might need some other lights, but it allows you to expand your lights one at a time for around $100. That is what I'm planning on doing, as soon as I have the $100....hahaha... One thing that you might also be able to do, if you don't want your on camera flash to "fire". You can put a piece of non-exposed but developed E-6 (slide) film over the flash. It will kill the visible output of the flash, but will allow the infrared range to pass through. The flash slaves are sensitive to infrared, so it fires the slave flashes without your on camera flash. Ahh, the power of Google.

I just bought a Canon A70 for $269. You can purchase a housing built specific for it made by Canon for around $200. Not bad for a 3.2mp camera and underwater for under $500. (bhphotovideo.com is where I got them)