There are a couple of ways to look at this. If you love your frames and the optician says that they are a suitable candidate for new lenses, go for it. Metal frames are probably more likely to deal with the stress of getting new lenses. If you really just want to save money by not having to purchase frames, you may end up paying more.
Most frames and lenses actually come from the same source. The fancy store you are in is probably selling the same lenses as popular chain stores. There is nothing wrong with that, but be aware that you may be able to use this to your advantage. A reputable store will offer price matching on frames and markup the cost of the frames to make it cheaper to buy them together. It costs more for them to have to send in your frames and have the lenses made for the frames than if they use a pair that is already in circulation.
Eyewear has become a fashion statement, hence the price increase. If you don’t really care about that, you can find inexpensive, yet durable frames and go nuts on the lenses. Yes, it is cheaper for the retailer to have them fitted with the total kit, but it is also good business to not let you walk out with a pair of glasses that aren’t going to last long. No retailer wants their customers to say that their product didn’t last.
If your glasses are made of plastic, they are more likely to be damaged when the optician examines them. It is the nature of plastic. It was not made to withstand heat and sun all the time. The best plastic frames will eventually give. Metal frames can break as well, but the repair will likely be a screw on the hinge or nose piece. It is possible to bend them beyond compare and even to snap them, but it is more difficult.
Do consider that opticians are not trying to make you spend more. If you absolutely insist, they will replace your lenses in your old frames. They should tell you that they are not responsible for any damage when they do. If they don't, walk away. You can always follow sales and clearances looking for a good deal. You will probably have to wait just as long.