You don't want to route a thick cable like the main over the cards. Looks like crap and it's in the way.
As for reaching, it depends on what psu you use. Some have a long main cable while others are a little short. Antec's own psu's take in to consideration the psu being in the bottom of the case so the main cable is a tad bit longer. But they're not cheap either.
Normally the psu fan points down from the top mount and the cables come out off the right back side. When mounted on the bottom of a case the psu is inverted making the cables come off the back on the left side.
The 180 series has the psu sitting up off the bottom of the case in a lock down bracket providing an air gap under the psu. This also allows the psu to be mounted fan down so the cable comes out like it would if it was on top of the case. At least my P182SE is that way.
In the case of the 900 the psu sits flat on the bottom inverted. So that alone would make me either not use that case or find a psu with a long main lead.
As for the shipping being more than the product, that's because most online companies have a minimum order policy. Check local shops. I'm sure you'll find the extender at one.