By the magazine, I'd say it was a HK SL-6 (.223). Unfortunately, they do not accept HK-93 mags nor does the SL-7 accept HK-91 mags (the latch system is totally different).
They're priced high if you go to a rapist dealer. When I sold my SL-6, a dealer wanted to pay me $200 for mine because as he said, "there's really no market for them." Thanks but no thanks. Two months later, he had a SL-6 on his rack for $1200. Apparently screwing faithful customers was just par for the course.
Keep your eyes open for a private party sale and you could probably get one for around $500 (that's about what I paid at a decent dealer).