"isn't quite expensive" ??? Do you know the second hand (or new) price of a Mellotron ? Unless you get the (VST software) M-Tron or the Manikin Memotron (still $2000...). And after all: the Mellotron contains not the sound you're looking for (MOOG) - some Mellotron tapes have a MOOG sound but these are awful samples, and you don't have e.g. the typical MOOG filter.
If you can afford something between $500-$800, then a (real) MOOG Micromoog or MOOG Prodigy would be a far better idea.
If you can't afford one of these, I can only recommend software; the Arturia Minimoog V is a very nice software emulation; it doesn't have exactly the same sound, but it can come close.