I am at 10 lvl now, 4-5 upgrade for PC costs 12.92k shares.
Definitly good upgrade, but there is for example deco which improves key drop rate, and it is cheap, it gives +1% per level and 7-8 upgrade costs only 848 shares.
More keys = more time skips = more gold transactions.
Diamond = less keys = less transactions, but more profit from each.
Hard call there