Somethings worth is totally dependent on both the buyer and seller, so it's hard to give a fair "price". It also depends on if you're trading for other Pokemon or selling for Pokedollars. We do have a Discord channel which I will provide the link where you can help trade for other Pokemon or even get the up to date per pokedollar value, it's not a set value but more of a guide. https://discord.gg/pokemonvortex
As for the Genesect and it's forms, it's mostly for collection, they all share the same type and have very little notice in looks. So one won't be stronger than the other, the only real difference besides a slight look (In the drive on the backside) is the move Techno Blast, Blaze form will use Techno Blast (Fire), while Lightning will use Electric and so on.