The following pieces of equipment offer cheaper alternatives to purchasing a dedicated performance computer:
- Use the house stereo (if available)
- Use your smartphone (if available)
- Use your dumbphone (if you still have one)
- Use your tablet (if available)
- Purchase a candy bar-style digital recorder and use it as your device (I do this personally and it works fine.)
Pat also brings up a good point: a stereo recording played through your amp won't do the world any good. It's also not going to provide a very wide stereo field for people to listen to. I'd advise you to use the house's amps whenever available and set them up wide to give you a full sound. If you own your own PA setup, then yes, I'd lug that around if it gives your backing tracks the presence they require.
To clarify, my equipment recommendations were made with the thought that you'd be plugging an audio out into a larger set of speakers for performance.