Generic PVR Remotes
Contrary to what might be expected, it actually doesn't take a very fancy remote to control your Myth box. Quite often your local department store will have one or more generic remote controls (sometimes made by One For All) which are designed to control multiple devices including a PVR, and will have several of the PVR-specific function buttons (like 30sec skip, instant replay, approve/disapprove, etc).
Some inexpensive remotes, mostly sold by One for All and Radio Shack, are actually programmable, when paired with appropriate software and a connecting cable. These remotes include a connector, often marked "JP1," that enable you to reconfigure and expand them via computer software. See the JP1 universal remote controls (One-for-all) page for details and additional links.
The One For All URC-6131 (and the URC-6131nwB00, don't be scared by the number change, it's mainly a cosmetic difference) is a remote that is not normally JP1-programmable like many of the other One For All models, but can instead be programmed through keypresses to remap buttons for other remotes it knows the codes for, to whatever buttons you like. These remotes typically cost less than $15USD making them an excellent "budget" choice. Joe Votour at The Vulture's Nest has a good explanation of how to do this, as well as two nicely commented lirc configuration files ready for use with MythTV. It's possible to add a JP1 connector to the URC-6131, but doing so requires soldering skills.