This will vary massively.
Not only because of massively different budgets, but also different sized families and also different ways of thinking.
Also, IME, people who post on "how much do you spend on your dcs' presents at Christmas / Birthday?" threads seemed massively skewed towards people who spend a lot.
Seriously, my 3 have got to adulthood and we've only ever spent anywhere near £100 when it has been a sort of 'one off, something they needed anyway' type present, like a laptop when theirs broke in 6th form, or a musical instrument that was still between birthday and Christmas.
If your dc has Grandparents / Aunts / Uncles / Godparents buying for them, then (in my opinion) you'd be as well letting them get the presents and you putting some money away for them. There is no need to spend £££ because you feel you ought to
Of course, if you are loaded, then you can spend whatever you wants, but you really, really, really, do not have to worry about this :
He's off to nursery in September so would like to gauge what the 'norm' is as inevitably he will chat about it.

a) he won't remember
b) no-one will ask
c) no 3 yr old has any concept of cost of an item and is as likely to be more excited about a sparkly notebook from poundland than anything expensive even if their birthday were that morning.