I recently bought a relatively cheap but decent XLR mic for vocals (Shure PG-48), along with a XLR-> unbalanced 1/4" TRS cable and a 1/4"->1/8" adapter. Recording from this using my Audigy results in something which has to have about 40dB of gain added in software and thus ends up with a rather high noise floor. (This is not due to an impedance mismatch- the Audigy can handle a 600 ohm mic- but rather to the mic's low output level.)

So, any suggestions on how I can find something inexpensive which will amplify a mono signal by 40dB or so?

The M-Audio MobilePre should do the trick. I don't think inexpensive mono preamps exist.

what about this one

For the price of a lot of the preamps I've seen, I could get a pro audio card with XLR inputs. Spending that much on something like this just doesn't make any sense.

A little more looking around shows that a high end chip which does all of the work costs less than $5, while less flexible/featureful chips with higher THD and noise which could still do the job well enough can be had for under a dollar. Surely a power source, a couple of connectors, a knob, a tiny and simple PCB, and some kind of casing can't account for a hundred dollars of cost.

If you are handy with a soldering iron, Rod Elliot's Low Noise Balanced Mic Preamp would do the job nicely. If you do choose to use the design, you can (and should) buy a PCB from Rod. Not only does this make your life easy, it also supports his site.

The reason this stuff is so expensive is because it has always been the preserve of recording studios and audiophiles. Sure you can make the things cheap but as there was no consumer market, who would bother?
Until the interest in home recording took off it was a small and very demanding market and alot of the models were hand built with a great deal if component testing and selecting done to make sure only the lowest noise and best channel matching was achieved.

This is equally true of phono pramps where consumer models have started to appear lately.

UJ

Yo can buy a cheap Behringer mixer which includes a couple of mic inputs with gain of acceptable quality. There are models which cost less than 100 €. Look at the UB series.

I recently had a similar problem trying to record my daughter playing the piano for an audition CD to be submitted with an application to a summer music camp. I looked online for some inexpensive mic preamps, and I have my eyes on the following ART model (haven't bought it yet) ...

ART Tube MP Preamp

These things sell for under $50. Is that within your price range?

If you just get a preamp rather than a sound card with internal preamp you will also need to replace your Audigy unless you want really bad quality.