What's to be addressed is that these numbers don't match the parameters in the HandicapInfo XML file.
I am however seeing mostly the same thing at Deity. Units and most improvements are costing 48% for the AI. The few wonders I've checked are at 80%. Since the modifier for Deity is supposed to be 60% for units and 100% for wonders, there seems to be an 80% modifier applied across everything. It's curiously inconsistent though - an industrial park for instance costs the AI 252 hammers vs. 600 for me, so 42% in that case. There are a number of others which are close to, but not exactly at 48%, which suggests we may be dealing with multiple factors, and which are suffering rounding errors (as hammer costs do not retain fractions). Even more confusingly, the costs are not the same from AI to AI. For instance an observatory was costing one AI 202 hammers, but another only 189, at the same time in the same game.
I ran some checks at Immortal as well. The Taj Mahal clocks in at 2100 hammers for the human, but 1848 for the AI - so curiously they're getting a 12% discount there. Given it scales logically with difficulty, this suggests it is deliberate. However there is no parameter with this modifier in the entire handicap file, which is somewhat baffling.
EDIT: I may have found where these modifiers come from. There are a set of parameters in the CIV4EraInfos XML file, which have the same Create, Construct, Train lines as the HandicapInfo file. On examination, these go from all at 100% for the ancient era section to 60-80% for the future era. I think these modifiers are being applied on top of the handicap bonus for the AI.
For example, the industrial park costs me 600 hammers in any era. The AI in the modern era was getting a cost of 252. 600*0.6 = 360. The constructpercent modifier for the modern era in the era file is 0.7. 360*0.7 = 252, i.e. the cost I'm seeing for the AI in the game.
The era is definitely relevant - in the earlier case where one AI was paying 202 for an observatory and another 189, I checked the era. The 202 one was in the industrial age whereas the 189 one was in the modern age. Gifting tech to the industrial AI to move them to the modern age changed their observatory costs to 189 as well.
So it looks like there's another set of modifiers for the AI in the Era file, which reduce their costs as they progress through the eras. This effect is only present for levels above Noble, and is partly specified by the AIPerEraModifier line in the Handicap file (I'd wondered what that one did). It isn't seen at Noble or below, where this line is set to 0, but is seen at all higher levels, where it rangess from -1 to -5. Setting it to 0 in the handicap file removes all discrepancies - e.g. after this change industrial parks now cost 360 for all AIs at Deity, and observatories cost 270 for all AIs regardless of era.
This is quite a sneaky modifier - since it is zero in the ancient era it easily slips through basic world builder checks which will usually just be built from the starting position.