I've tried gluing parts of velcro that were broken off back again on the pad of my Rotex, but it was futile. As soon as I had to replace the sandpaper it would just rip the glued part off again.
Yes, the pads are ridiculously expensive, and I don't like to spend the money to buy a new one either, but I've grudgingly accepted it as the cost of doing business.
Unlike the Rotex, which has a proprietary mount, the ETS 150 has a very common standard hex bolt in the middle. Maybe it is possible to find a replacement pad from another brand that might fit and is cheaper. But gluing something on, well, that seems a recipe for failure to me.