That's on par with costs from custom shops.
Keep in mind, this is not an individual doing this on the side... with them making all the money on it. It's not even a individual who does this as a business of sorts.
It's a retail shop.
If it takes them a day to to build the box. Figure $50 in materials (wood, glues, staples/screws, caulk, carpet). Paying a guy $15/hr or commission to build the box (so, ~$120 for the employee building it). Then the shop needs to make at least equal that for it to be worthwhile... and to go towards tool upkeep and investment. So figure ~$250 labor and $50 materials.
That's the deal with B&M shops. For a ported box for a pair of 12's... carpeted... at my shop in would b $200-300. I build boxes rather well and quickly though.
nG