Relatively inexpensive with pro features
These appear to be the cheapest MIDI to USB cables on eBay that work well.
I tried one of the cheapie ones before buying this, but that used a non-standard circuit which does not isolate the devices electronically. It also ignored entire classes of events (only handling key-on, key-off and simple clock events), and then dropped half of the events it was trying to respond to (so keys got stuck open, because the key-off event was dropped by the cable et cetera). It was useless, and other people have reported similar experiences.
This product is more expensive, but it works perfectly. It faithfully carries every event (of every kind) with low-latency. The build quality and design are pretty nice too (for what it is).
Verified purchase: YesCondition: New