Hi Lucky,
If you're looking for price comparisons, visit various reputable pen shops (online and brick & mortars) that are always mentioned in the Pelikan threads on FPN.
As for me, I bought a new M400 White Tortoise on clearance at Swisher Pens for about $140. Swisher Pens also sells the Black/Black and Black/Color varieties for about $30 dollars more.
Richard Binder has already been mentioned, but I'll also mention Pendemonium and Nibs.com (has some beautiful of varieties of the M4xx series). Also, Todd at isellpens.com gets mentioned a lot for other brands (mainly the Hero, Duke, and other off-brands), but the price of new M400s at the site are relatively low at $155. His stock at the moment (I just checked for this post) is limited to the Black/Red and Black/Green however.
I'm not affiliated with any of the shops mentioned, but I recently bought a Pelikan M200 and another pen from Richard with two specialty nibs. I've bought pens from isellpens.com several times, but not a Pelikan.
As far as eBay, I limit my eBay bidding/buying to NOS and vintage pens and can't comment on what a reasonable price would be for a new M400. You may want to search the Pelikan and Writing Instruments forums on reputable eBay sellers who sell new Pelikans. I've seen a few sellers mentioned from time to time, but off the top of my head cannot remember the sellers IDs.
Oh, and one last thing, I always factor in the shipping cost. Sometimes it coincides with the items that I buy (weight, destination, and so forth) but often, not. It seems that shipping costs on eBay are pretty random. The lowest price you find might also have a very high shipping cost, especially if it's coming from overseas.
Hope that helps you. Happy hunting/shopping...