Hi Michael,
This depends on what sort of music you are recording. I have used a few drum machines, but find most just don't sound that authentic, although by all accounts the Alesis SR18 is supposed to be quite good at a reasonable price. What I am currently using is PC Drummer Pro. This is a great piece of software built on very good drum samples. It is really easy to write percussion on and you can even save and print the drum tab to give to a real drummer to play what you've written and recorded.
This software is currently $49 USD with the standard 50 instrument kit. You can also buy additional kits for $10 with quite a range such as Jazz, Hip Hop and more. I purchased the Studio Kit which has 408 sampled sounds, multiple kicks, snares, toms, cymbals and effects. It also comes with Bass and Piano samples for all notes.
This produces a much better result than any drum machine and depending on how much effort you put into creating the drum line, most people wouldn't be able to tell the difference between this and a real drummer recorded.
You can adjust the volume/intensity on every sample at every beat and the humanization feature allows you to vary the hit levels and timing from perfect to awful, so you can mimic a great drummer or even a beginner.
When you have completed the song, you can save it and also export each instrument separately as a wav file in mono or stereo. As i use the Tascam DP 32, I transfer the files separately so I can mix each channel independently as if recording a real kit with multiple mics.
Enough said. Check out the website. You can download a free test version and see if it fits the bill. A lot cheaper than a drum machine and much more authentic.
PC Drummer - Create Professional Drum Tracks