If the cost for maintaining the infrastructure in 10x that, then it's a shoestring budget. You have to compare the budget to a baseline, you can't just evaluate it as an absolute. Yes, £10B looks like a big number, but what do you compare it to?
- If your budget is $10 but it costs $100, that's shoestring.
- If your budget is $10 but it costs $1, that's not shoestring.
You need to know both values; shoestring is relative.
Other large cities with comparable transport infrastructure?
You, or any passing reader, can just spend 30 seconds with a search engine and find dozens of papers, reports, presentations, etc... doing this kind of financial comparison.
You might as well submit replies saying that rivers flow downhill instead of uphill.