I think we’re talking slightly at cross purposes; the Di2 upgrade kit is/was about 2/3 of the price of the full Di2 groupset.
So before Covid, in the UK, a full Ultegra disc groupset would have cost you about £900, a full UDi2 about £1800. If you bought Ultegra then upgraded, your spend would have been (very roughly) £900+£1300, so about £2200. Factor in probably about £100-150 labour if you’re getting a shop to do it, the increase was probably about 25%-30% over just getting Di2 in the first place. So not insignificant, but not 65-70% more expensive.
This is all academic, though, as I think Di2 parts are very hard to get at the moment, and given that mech DA and UDi2 have always been very similar money, I’d always choose UDi2.