Depends on whether it's a reissue, vintage, or loose. I paid $120 for a boxed reissue in a long box about three years ago, and the prices for that are still about the same. Linky to a reissue.
This one is a little low, but it's been opened and the stickers are applied. This is the version I have, with the box I can't read and everything.
The book style box (white box) goes for about the same or more, and if I was going to buy a G1 Megatron, that's the one I'd look for since it has extra accessories with it. Example.
Loose vintage can be had for around $50, if you don't get into a bidding war. Linky.
Boxed vintage is pretty much astronomical, IMO. Dig this
auction. It's an American version, with an opening bid of $300. There are few Transformers I would pay that much for. I'd pay that much for a loose complete Fortress Maximus, but last time I checked, they go for more than $300.
I like this
auction. If I had the money to compete for it, I'd throw my hat into the ring.
Megatron is a little easier to purchase than Optimus Prime. I've seen Hasbro's 2002 reissue get towards $200 MIB, which is insane. Takara's New Year Prime tends to get a little nuts too, as well as the white boxed version that's a couple years old.