I own the fore mentioned Prestigio Roadrunner 520 and I can say that it is small 5x4cm, wide lens, full hd, records in loop on a 32gb microsd ( no need for any "unit" dvr whatever) easy to mount/dismount. It was 100euro.
I noticed the type that the dash camera embedded in rearview mirror, very covert and others can hardly discover it, many taxies have been equipped with this type already, I think the price won't be too expensive .. :yum: