Weber is not sold at Byggmax but at more professional stores where discounts are all that matter. The same bag you pay 80kr for, we buy for 35kr at places like Optimera, and it's incredibly unfair, but I guess they want to penalize private individuals who find their way into the store.
K-rauta, however, is a middle ground as they charge about 60kr for the bag, with a 10% discount if you're a member. But if you don't have a K-rauta nearby, just find a masonry company in town and say you work there but want products for home use and will pay privately, so you'll at least get their discount.
Regarding the question, the cheaper ones feel a bit different to use, and it depends on the quality of the sand and lime. The cheapest mix might be a bit stiffer, but there are absolutely no technical construction issues using them instead.
However, don't use C-mortar on a foundation, at least B, but preferably A.
