Nothing is etched in stone. Perhaps the frame sizes... Deeps can be cut down to mediums. Even the frames can be cut down if you have the time and ability. I have done a lot of them.
Price for Deep vs Medium is really only a factor if you are going to have more than five or six hives. After that it starts to add up...
The medium box will be easier to get wood for and cheaper, but you need ten more frames. While a medium frame is cheaper than a deep frame, you will need thirty instead of twenty.
I run all mediums, even though I am pushing close to 100 hives now just because I am getting older, and as stated, because i like the interchangeability. I can pull frames from supers and drop them in hives to help them be ready for winter. I can also use extracted honey frames in the brood chambers to start new hives and give the bees a head start. I can out anything anywhere I need it. i do not have to worry if I am adding supers and realize i need to replace a deep. To me, it is just about convenience.
If you run tenish hives, will the extra hundred dollars for the frames be an issue?
If your running mediums and decide to move to deeps, you pull a medium and replace it with the deep, and as it is drawn out you move it down and add another deep. By the end of the year you should be able to pull the last medium. It works the same the other way around. If they are using it, leave it on till spring and then pull it when it is empty. No emergency and no rush.. thats part of the joy in beekeeping. Unless you have a swarm issuing from one of your hives, you can move slowly and enjoy what your doing.
The swarm? Well, at least for me thats a lights and siren emergency!
I have needed a deep and only had a medium, so glued and nailed a shim to the bottom of a medium to MAKE it a deep, and I have cut down a lot of deeps into mediums...
There is no right answer to give you, the right answer is what YOU think you want to go with, all the rest of us can do on this issue is tell you what we do and why. The rest is in your capable hands!
Scott