I'd recommend just going USB to USB, rather than trying to go from USB MIDI to DIN MIDI (the traditional 5-pin MIDI connection provided as a courtesy on many audio interfaces). While it may be possible to find a cheap adaptor with the right connectors, that may or may not workâ€”AFAICT most of them are intended for converting MIDI from DIN to USB rather than the other way around. If you feel you really need to do this, you'll want to do a little research, maybe contact the manufacturer of whatever device you're thinking of using first to see if it supports what you want to do.
In general, MIDI over traditional DIN cables is considerably slower than over USBâ€”if you're just playing a keyboard or drum pads an older slower MIDI connection will be just fine, but if the MIDI data includes lots of continuous (controller) data you're better off sticking with a faster USB-USB connection.
Here's a couple of links.. The first is to a page on the MIDI Association websiteâ€”the video at the bottom talks in detail about MIDI/USB adaptors, maybe you'll find it helpful. The second link is to a webpage for a MIDI/USB conversion box from a known manufacturer (Kenton) that seems to do what you want, though it'd cost *way* more than $15 (more like around10x as much!).