Tooo late :D You upgrade from the most expensive to the less one... because the price increases the more you buy. So there is an efficiency ratio.

If you divide the honey/day for the cost of upgrade (example, Wax-O-Bee is 450/1121=0.40), you can find the upgrade that gives you more honey at all times.

The Nec-T-Bee first upgrade is 3150/1000=3.15 vs the 0.4 on the Wax-O-Bee, makes it way more efficient to wait for buying the Nec-T-Bee.

But obviously there is a limit on how long you should wait... too much time waiting loses efficiency too.