I know AEA used to make one, but Wes pretty much dropped all projects other than strictly Ribbon mics, but you can probably find one used somewhere for fairly not expensive...
Millennia and Dangerous make expensive ones, but obviously those won't work...
Most field mixers have m/s decoding built in. I know the Wendt X5, Sound Devices 302, and PSC ProMix 6 all have it. There may be a less expensive mixer that has it built in, you might be able to find one, or something used (on Coffey Sound or Trew Audio maybe?)
I haven't seen a stand alone decoder under a grand though.
Alternatively, you could use a y-split XLR cable to go into 2 channels (on your side channel) and use 3 channels and decode manually. You could either phase reverse the splitter manually or phase reverse on your inputs if your mixer supports it.
Good luck!