If so, what would you be interested in charging to build such a box?
Depends on how elaborate you want the box to be and how difficult you want it be to defeat the lock.
Perhaps it would be better if you looked at the electrical hardware requirements, priced them out, and then proposed a fee for programming/building the box.
Should the box show when it can be opened? Or how long until it can be opened?
An Arduino, a RTC, a battery and wallwart, an LCD, a few switches, and a servo would be all the electrical hardware needed. Some box with a hinged lid and a latch that can only be opened by the servo (or a key) would be needed on the mechanical side.
The programming is relatively straightforward. Set the clock (current date/time). Set the opening time/date. In loop(), see if it's time to open the lock. If so, rotate the servo 90 degrees.
Going to sleep, and waking up periodically, to check the time, could extend battery life.