lynolyn
do you have the other details from the GIA cert? e.g. table, depth, girdle, polish, symmetry, fluorescense. should look at all these in relation to the price.
thru my own research, ideal depth is btwn 58-60%, table size, 53-57%, for ideal diamonds, polish and symmetry should be very good, if not excellent. girdle shouldn't be too thick or too thin, but for solitaire ring, the girdle can't be thin cos may chip off easily. fluroresence in good colour diamonds is not good, but will enhance colour for colours I and above.
d colour is very very white, of cos it will make a difference i believe even to an untrained eye. any jeweller should be able to advice you that as your rock gets bigger, the colour will become more apparent, unless you deliberately prefer a coloured stone.
hope this helps...but you should do your own research...sorry i can't advice on the price cos i've not been looking at 50 pointers.