i could be wrong, but i don't think his memorial exists anymore. Ella asked that it be built (i don't know if she or the state paid for it) and it was one of the first things destroyed by the revolutionary government (Serge was liked by very few).
also, i too am curious as to why Serge Aleksandrovitch's body was not re-buried in the Romanov crypt? maybe it boils down to money & publicity? reburying his body with even remotely similar equippage as those already resting there, would cost an enormous amount of money. and the world at large would have no clue whatsoever who he was; but reburying Nicholas II, Aleksandra, & family, while outrageously expensive, would (and did) reap even larger benefits in terms of publicity & revenue through various means.
but that's purely speculation, and he may be there resting peacefully.
does anyone know for sure?