giving the customer the amount of spent effort

Hi all!
Keeping in mind that the screen customization module exists, so probably using it to get what I'm going to ask should be possible, here first some:
PREMISES
Let's define Product assistance projects as time-capped (usually monthly assigned but yearly planned) contractualized pre-paid activities on which the customer opens some ticket, asking for time engagement on that in respect of some SLA. The resource will work on some related (linked) activities in the project but also he/she would possibly consume time directly on the ticket, without plan needing (let's say, for very small tasks).
Now, usually a customer buys this kind of assistance not in form of time but in form of, let's call them "coupons". Strictly speaking there's an equivalence between time and coupons (eg.: 1 coupon = 0.25hr or things like that), but this is not completely true because you can have different seniorities and differeent kinds of engagement on tickets (a 1st level helpdesk is not like a development task), so a "coupon translation table" (per customer, per resource , per ticket type and maybe per period/version, just for history) should exist, but this is not so important for the moment. 
Said that, here's the
QUESTION
how to show the total amount of planned coupons and consumed coupons to the customer for the ticket that he/she opened? No real effort in terms of time neither cost. Only coupons count in this case. I was imaging a pair of fields into tickets and simple tickets screens. This could be very useful because the customer periodically can ask for a report such as: number of coupons spent this month on tickets, trend of coupons consumption, and things like that.
Obviously, the best approach could be to have that coupon translation table, so in any moment the translation between effort and spent coupons is not only possible, but also automatic, as the result will vary depending on who's the customer, who's the resource, which kind of ticket type the resource is treating and lastly which version of the table is using (for history, as I said before).
If this table must be a built-in structure, so a perso occurrs in this case, then at least it would be nice to know if these two new additional fields can be filled syncing data captured from another software (through a cron job?). REST API calls are possible on the other side.
Best wishes!

Hi babynus!
Great to know it! Thank you!
Best wishes!