That can still be obtained (I'm reliably told) in parts of the country for £20-£25 a week or so without help with jobs. So £5k-£6k for initial set up and maybe £1500 a year thereafter for other costs (not including lessons) as a very minimum.
I pay just over £150 a month for my basic rate livery at a yard with stable, field and it includes all hay and straw. I'm in a cheap part of the country, and I'm very lucky with the yard and its facilities.
But that's nowhere near the total of my costs.
I do have insurance, which costs me £80 a month. I know that some people don't bother, but I don't have the savings to cover a huge vet bill. I do also have a credit card for emergency vet bills too!
My pony has fronts only, which costs me £40 every six weeks (ish). Obviously some can manage barefoot, but an inexperienced owner would still need to pay for trims, which aren't loads cheaper in my experience.
I do give a tiny amount of hard feed- this isn't essential, but all the horses on the yard get a feed in the evening, and he gets distressed if he gets nothing. This costs probably just under £10 a month (could possibly be done for less but I want him to get a good quality feed, and does include some treats).
There are other, more incidental costs- such as teeth/vaccinations/wormer etc. I try to save £30 a month towards these.
I reckon my very, very basic yearly costs are about £3500.
I agree with you that DIY livery is a pretty cost effective way of doing things, and it's great that you get facilities provided, and it probably does save me money in terms of maintenance and bills (e.g. providing water at the field etc).
I'm not sure grass livery is any cheaper these days as land is expensive and bringing them in at night means more horses can be kept on less land.
I'm in West Cornwall, so land etc is relatively cheap here. I would guess there's no way OP could get livery for this sort of price in e.g. the SE.