How about zero dollars? How technical are you?
If you're so inclined, there's a firmware "hack" out there for Powershot cameras (which your A540 is) that enables a whole slew of features, including manual controls, making your existing camera pretty competent for taking aquarium photos.
It's really not a "hack" in that you are making permanent changes to your camera's firmware. You load CHDK onto your SD card, and set your camera up so it boots from the SD card instead of its firmware. That's really it. You can revert back at anytime. It doesn't hurt the camera's warranty, but you're warranty's already expired by now anyway.
The features that CHDK enables that are invaluable in taking macro photos are RAW support, precise manual focus, and the histogram. Yes, your camera supports them. Canon just decided to disable them on your model.
With the precise manual focus, you're no longer dependent on the camera's easily-confused autofocus system, and you can adjust the focus to the . And with the histogram, you can check your exposure.
I used an A590 for awhile before I finally decided to go with a dSLR. It's got limitations, of course. Specifically, it's hard to check critical focus on the LCD because it is so small, and it's hard to get good quality pics of things further than about 8"-10" from the glass. There's too much refraction.
If you're interested, check here:
http://chdk.wikia.com/wiki/CHDK
Here are a few samples of the pics I took with my "hacked" A590:
