Although I'm not an expert, I've seen them selling for around $275-$350 in condition similar to your gun.
The M51 definitely has a following. IMHO it's one of the neatest vintage .380s out there, surpassed only by the J.P. Sauer & Sohn 38H.
I would recommend reposting this in the "Curio & Relic" subforum. Several regular visitors there are vintage small-caliber pistol enthusiasts.
FWIW the M51 actually has a rather intricate design; although it appears externally and functionally similar to the striker-fired Browning FN Model 1910 and 1922, the M51 actually uses an internal hammer.