You may inadvertently spend more than you have to - there are some carbs that cost that for B&S motors currently in tractors, but not many that I have come across, and rebuild kits most often cost less than that. We recently put a carb on one of our vanguard Vtwins that cost us (authorized B&S dealer, through our supply chain) $150. Point is make sure you do some homework before just jumping onto eBay and plugging in the part numbers you found online.
There are also literally hundreds of combinations of parts that can apply to a single engine size/family, so be sure you have your engine numbers and use them to get the parts. Just searching for a part for part for your engine (or tractor/mower/whatever) without them could end up with a useless part, or a workable but incorrect part that could actually lead to damaging the engine.
Stens is overpriced. True Blue is their branding, that's all from what I understand. Stens does not make belts, nor are their belts specially made for them. If you're not picky about OEM Oregon made is as good or better and much less expensive.
There are several v-belt manufacturers, but in all reality most come from Dayco, Goodyear, and Gates regardless of whose name is on the card sleeve. Belts are mostly standard sizes - width and length. Most manufacturers of equipment (power equipment, automobiles, industrial machinery, whatever) will design their product to use a standard size belt because it costs them much less to purchase the belts versus specing a non standard size that has to be made for them. They may have their own part number and packaging (for replacement parts to customers) that do not reflect or state the standards and they may even have deals to get the belts unmarked, or stamped/marked with their own name/part number.
companies that do use custom (Toro is the #1 in my mind in my field) do so for the replacement parts revenue. for most of their equipment the belts are only available through them (again not made by them, but made for them with exclusivity).