While its not possible to own an Evangelion mecha, you can still buy two new Evangelion-themed bicycles designed personally by Yoshiyuki Sadamoto [貞本 義行], the character designer for the Evangelion series.
The bicycles (pictured aboved) are called the EVA00BIKE type-r and EVA02BIKE type-a, and are produced by Media Magic. As you can see, the color schemes are inspired by Rei and Asuka. The bicycles are scheduled for release in Jun 2010 and will each retail for 98,280 yen.
Source: 自転車補完計画 via Canned Dogs
I don’t think I would fork out so much money for just a modded bicycle with a Nerv seat. They should have at least bundled it together with a cycling suit and helmet to go along with the bicycle.
I’ve seen better bikes that are worth the expensive price.