As latreche34 says, the BE-75's are usually pretty spendy and rare. But they do sometimes appear on
eBay pretty cheaply if you have lots of patience. Have you considered the BE-1 or BE-3? These are basically the BE-75 less the audio portion so you'd have to capture the audio with another device. They are usually cheaper the the 75's that come up and also seem to appear more frequently.
You also have to get the SDI stream into the PC somehow for storing as AVI. For the BE-1 and 3, you can solve both the audio input and SDI handling with a single device, say a Matrox MX-02 LE or an AJA Kona LHe with breakout box. These both are a bit clumsy and need more pieces to complete the signal path, but it can be done. I've tried both and quick testing looks promising. You could also combine a BE-1 or 3 with a BE-20, 21 or 23 (I THINK!!!) to embed the audio into the SDI like the BE-75 does itself.
For pure TBC use, you could pair BE-1 or BE-3 with a BE-11 and then route this back into your current analog capture devices. I have yet to try this but have the pieces to do so, just not the time, yet...
If you haven't looked it over,
here's the full BE catalog. There are other interesting things besides the BE-75.
Be aware that the BE-2 does NOT have TBC and the BE-25 and BE-26 are composite only (no s-video).
BW