Ah then I understand the background for TCG, maybe MCM/MKM would be more okay with it than TCG?
For me, I export my collection, run it through my own program that modifies the CSV file for certain sets, move the csv to my mobile phone, import it into Delver Lens, get a 10% fail rate of cards, and then I can see the price in MCM for 90% or so this is indeed very time consuming, and when trading a lot it makes it very hard to keep track.
Hmm... if TCG not allowing Deckbox to use their prices is the problem, a solution would be to create a secondary site, which just has one feature.
Let's call the new site a random name, say "money-dash-5".
The only feature of "monkey-dash-5" would be to import a CSV file, and calculate the price depending on MCM/MKM and TCG and show it.
And then deckbox just needs to add a button that says "export to monkey-dash-5" which would create the CSV file and use HTTP POST the file to "monkey-dash-5", and the user could see the value there.
Would just be a front with the same calculation logic as before, and parsing the CSV in the same way as Deckbox already does.
So from a developer perspective, it should not be super much work.
Of course, maybe the TCG integration is deleted, and building the MCM/MKM integration would be a lot of work. I have looked a bit at their APIs and they are workable but not super friendly