I think that really depends on the nursery...
Mine go to preschool, there is no required uniform but they can wear a jumper. I bought 2 @ £8.95 each and they are currently doing service for child number 2 and will be passed to child number 3 next year, so very small outlay.
They have a lot of fundraising events, I guess I put about £5 per month in for various raffle tickets and sponsored bits, I also bake cakes for cake sales etc.
They do do trips, one per year and usually costs about £10.
Thankyou presents - I make jam / chutney and it is always very well appreciated and cheap!
Book bags you can get off Amazon for £4 - my DD1's for school will probably last 2 years, it's in its second year and getting tatty so I envisage a new one next year - but that is a primary school pupil where they get significantly harder wear... I'm not exactly sure what she uses it for, but it is very battered!
I guess if on average you allowed £15 per month, that would be very generous and cover a lot, unless it is compulsory uniform and lots of trips.
But, like I say, it will depend on the nursery. When DD1 was at a day nursery as a baby there were no extra costs at all above a wee present at Christmas.
Last childminder was the same - all inclusive except for minor presents.
Current childminder charges £10 extra per week on top of fees during school holidays for ice creams / day trips / treats (she didn't ask, I offered as they love the trips out and I didn't want to see her out of pocket at all - means they all have a fab time and she can say yes to ice creams and small treats in shops without having to fund it). I think she is worth twice what I pay her anyway!