As Ilengine has already noted, not all parts are available aftermarket and each company will have a different price.
If you end up with a Stens account, see if you can get a hard copy of their catalogue, it is a lot faster & easier to use than the computer one.
This is particularly the case when looking for belts, blades & pulleys as it has a table of them by size.
With pulleys in particular, most now days have a 6000 series ball bearing ( although all catalogues show a double row bearing ) and the only difference between dozens of them is the bush in the center which presses out , and when you have your head around them, you will work out that in many cases you can substitute a more common pulley size for an uncommon thus very expensive pulley size .
That alone can save a lot of money.
Deere parts down here are about 1/2 to 2/3 the price of other brands because their mower division piggy backs on the agricultural equipment warehouse .
SO I buy a lot of genuine JD parts retail cheaper than aftermarket parts wholesale .
Prime Line had the best price on cables , starters & clutches.
Stens are best for belts , pulleys , bar blades ( except the Gator style which are prone to premature flute failure ) & air filters .
Oregon naturally chain saw blades bars chain & sprockets plus Gator blades .
Rotary air filters , oil filters , fuel filter, fuel taps & MTD vari Drive belts .
Where the big discounts come in are the trade packs.
Stens do 12 Packs and Rotary do 36 packs in oil filters so it becomes viable to par it down to 3 or 4 common sizes & buy the big boxes.
Briggs panel filters come in 25 or 50 packs as do several fuel filters
We have swing back blades on walk behinds and the 100 pair trade packs bring the price down to under $ 1.00 pair
Comparred to $ 4-00 to $ $ 9-00 a pair for individual pairs.
I will do 100 to 200 blade changes in a good season