With Masterpiece, you're looking at the premier line, so anything that has been out for a while, you are going to pay a lot for as most of these do NOT go down in value, ever.
MP Optimus, I agree with everyone else... if you want cheap, wait for the YOTH Prime that is coming out. If you want authentic or size in-line with MP05 Megatron, then you have to bite the bullet and get MP01 or MP04 (with the trailer).
Speaking of the MP05 Megatron, you're going to pay a bunch for him, period. He will never be re-released in Noth America because he is a gun, so everything is imports and that gets pricey. The current retail for his recent re-release from Takara is about $230USD plus shipping, so finding him from for $250 Canadian from guys like our friend,
Nullray, is usually a deal because you'd pay more than $20 in shipping, exchange from USD to Canadian, and Customs.
Hope this helps.