$250 for glasses like that is very cheap. I also have bifocals, not the thinnest lenses either, IIRC they were one step up from the standard ones. A light frame but nothing special, the frame was like €100, the entire set of glasses was around €650. The lenses only have a cylinder in them; no prisms or anything like that. If you need more complicated or stronger/thin lenses they can easily go over €1000.
Even if you have the optional insurance for it, that doesn’t really help you. The amount they cover is basically the same amount you pay for the additional coverage. You’re better off putting the money in a savings account earmarked for your next set of glasses.
Also, if you need anything but the most basic single focus lenses without any cylinders or prisms, get them at a real optician. The online store can’t properly measure where to place the lenses in the frame (they need to be properly centered in front of your pupils).