Did my uni placement at a biomechanics lab specialising in custom orthoses and hope to get into it when I graduate.
The price you've been quoted seems about right
I
strongly reccomend you go direct to a lab that can assess you and manufacture your insoles inhouse -it means they can sort out any initial problems you may have very quickly and (hopefully) free of charge. The place I was at the podiatrists and biomechanists were trained to
be able to make a set ourselves as some people have problems adjusting to them initially it meant we could make on the fly adjustments until they were spot on. I'd guess about 80% of business came from other podiatrists (i.e. posting casts & pescription) and of couse this means if there's a problem you'd probably have to pay for your podiatrist's time and then wait for them in the post and they could come back still not perfect.
As well as the above a proper knowledge of the manufacturing process and materials used is also beneficial to make your pescription as good as it can be. 90% of out of house orders from podiatrists would use the same materials for every pescription and in some cases it'd be up to us to ring them up and suggest a more suitabe material for a particular problem/function.
If going somewhere that manufactures in house isn't an option try to go to either a podiatrist with some form biomechanics qualification or vis versa (though preferably a team with both).
Most places should be happy to give you a free 'chat' explaining what they will do and will give you an opportunity to ask any questions and show you around the lab.
You won't be able to use a pair of custom orthoses in both running shoes and smart shoes as they won't fit correctly so you may be advised to get 2 pairs (or want another if you feel they do wonders!). Some places offer a subscription type service that includes x amount of orthoses, conslutations, free recovering and often other offers on running shoes/sports equipment. Something to bear in mind as they can really save you a lot of money. Where I had my placement, over a year with 2 pairs of orthoses you'd save a few hundred £ on the orthoses alone.
Without breaking forum rules on medical advice the symptoms you've described sounds like the 'bread and butter' of issues that orthoses deal with so you should get some benefit
As has already been stated the actual pescription (the hard bit) should last upwards of 20 years and should come with a guarantee, yearly recovering should be offered free or for a minimal charge. Given that they last that long a over 20 years ~£15 a year for something that should reduce pain and help/prevent further chronic injury is worth it to most people.
For the kind of help it can give, when I was working there my favourite part (even more so that working with GB athletes and premiership football players) was an older woman who came in with a photo of herself from 3 years ago. It was her with a walking stick in each hand. I obviously didnt know her but she had come to thank the team because she could walk pain free and said it had changed her life (while welling up) -job satisfaction doesnt get much better than that.
For those asking, quite frequently parents take children to get orthotics privately as fast growing feet with a ~9 week turnaround don't go together too well

. Also when buying orthoses you should be looking to justify the price in the after service as well as the consultations and product itself.
As philstanbridge pointed out orthoses can also help with a lot of other related problems too!
Not that I really needed them, I made myself a nice pair of carbon fibre orthoses for my basketball shoes (free of charge of course

) comfiest thing ever (once you get used to them!)
My that was long lol. Hope it helps, any questions fire away.
The place I was at was in stoke called Sub-4 Biomechanics, very far from you but would highly reccomend it to anyone (and no I don't get commission despite what all that sounds like!)