If you need your glasses to fly then you need to carry an identical spare pair, as per the rubber stamp on your medical certificate.
Wearing glasses that your AME doesn't know about doesn't sound terribly clever. You might think that wearing low powered reading glasses so as to see the map better is a good idea, but what's it going to do for your distance vision? You simply don't know unless you've been tested wearing them.
Don't I recall from the air law course something about it being a requirement to notify the CAA when you start to need glasses? What they do about it is up to them, but one might guess that the medical certificate is suspended until you've had an AME check out your eyesight again, and you've got the optician's questionnaire filled in.
If you need near vision glasses to read the map and the paperwork but mustn't wear them for distance vision then taking them on and off can't be a good idea, and you'd want to check whether it was actually legal. The usual advice is to get bifocals with plain glass at the top. (Varifocals are allowed but not recommended by the CAA's optician chappie.)
Price: ah, yes. Well, I've had less than perfect eyesight since about age 7, and being able to see as well as possible is important to me; I don't care what it costs, being able to see takes precedence over anything else I might want to spend money on. I choose frames for functionality rather than fashion; the main requirement is for those clever hinges so that the side pieces can hinge outwards when something, like for instance a child, bashes you in the face - that way you avoid months of tedious journeys to the optician trying to get them bent back straight again. (If you don't realise your glasses have been bent out of shape you can get headaches which doctors don't understand, and they try to treat you with migrane pills because they haven't diagnosed the real cause. Needless to say this doesn't work).
So, on starting to fly again recently I discussed with the CAA what was needed and allowed in the way of glasses, then got my optician to make some bifocals (normally I switch between reading and distance vision glasses which obviously isn't on when flying), then got her to have another go when the first pair didn't turn out right. Then another visit for her to carry out some of the tests required for the optician's questionnaire. (It was with great glee that she told me she's retiring in July.)
So, cost around £450 for the two pairs of bifocals, which I'm only ever going to use in the aircraft. Not very many hours' flying. There really isn't any sane alternative to doing it properly.