Why can't I set the price at price at product variant?

why am I not able to define price at variant level - AX 2012 R3? 

i released the product master with 5 different variant (my dimension is at configuration level) - I noticed the following: 

I ticked the 'use cost price for variant' then AX allowed for item price entry (cost versions)

ok. defined purchase, cost price- activated 

Opened a PO, i expect to see the purchase price but its blank;  

defined sales price (activated) still AX doesn't show the price in SO! 

what could be the reason. 

I can define an item, define item price that shows in PO, SO!   but why not variant prices

what i want is that I have 3 variants (dimension group confi is ticked) - item is sold in 3  configs -  small, medium, large. 

so i want to maintain separate price for each!