There is no such animal. Good and cheap are somewhat exclusive.
On the low end are the Winmodems from Zoom, etc. Overall respectable
products if you don't mind missing perhaps one in 10 or 20 faxes with
various "reception errors". OK for home users and light business use when
you will generally expect faxes and will know if they don't come through.
Next are the hardware modems such as the USR 5610B and Multitech ZPX series.
Still under $US 100, but far superior in terms of reliability with perhaps 1
in 30 or 40 faxes with "reception errors". OK if a few missed faxes is
acceptable. Disasterous if this means hundreds or thousands of lost dollars
when a customer's order gets messed up.
Finally, there are Brooktrout cards, which I have not used, but apparently
are the gold standard.
Although many swear by external modems (and I have used plenty), I'm more in
favor of internal models - one less power adapter. I've never had to reboot
a Windows NT/2000/XP/2003 machine because an internal modem was locked up.
All depends on your needs, but in general, the more you pay, the more you
get.
Marc