I maybe didn't put my point across well enough.
What i mean is you cannot buy one particular size of tyre in every speed and load rating combination, can you ?
Speed ratings, in some cases: Yes.
Load Ratings, for each size there is usually a std + an XL.
You will also notice that for the same overall diameter, load ratings stay the same (or very close).
205/55R16 is rated at 91 for the std and 94 for the XL. There are also specials, ie 89 for old 911 fronts.
225/45R17 is rated at 91 for the std and 94 for the XL
225/50R16 are 92
and in this example, 205/55R16 is available in at least H, V, W, Y for summer and lower for winter.
As for price, there is very little, if any, difference between any of the different speed ratings for comparable tyres. So you rarely save money by fitting under spec'd tyres.
When you get up to 18" & 19" sizes, there is no point tyre manufactures producing low speed rated versions as there won't be any vehicles to suit them (other than Chav Nova...).
There are obviously lower speed rated winter tyres and they are allowable (strictly you need a notice on the dash warning that lower speed tyres are fitted).
The tyre requirement is to cope with what the car is capable of, not what you intend to drive at.