If the card is a Sabrent SBT-TV FM, the chipset Tigerdirect mentions is apparently Linux-compatible - at least, it shows up in lists of mythtv-compatible tuners. As to whether that particular card works with Linux, I plead ignorance.
Hardware encoding for under 50 bucks? Not likely?
If the Sabrent turns out not to work for you, and you're willing to go with software encoding, the least expensive option I'm aware of is at surpluscomputers.net.
They have Win-TV GO PCI tuner cards for $19.95 (shipping included).
I've installed two of these on my home Linux boxen, and one on my mom's Linux system (all running Fedora Core 3 and MythTV (atrpms.net version)). They work fine, aside form the occasional lockup (gotta keep the capture cards cool).