I never paid for anything with my ex. I was an undergrad with no money and she was a post-grad (with some funding at least!)
Once things pick up, it makes sense to surprise her (like a silly rhyming line on paper wrapped around her favourite chocolate bar and posted to her or a bunch of flowers) but the value there is the thought, not the 60p (obv).
If the payment question pops up awkwardly on mutual occasions like dinner or movies, I think rounds - what chop147 said - is the way to go.
If she's got a job or anywhere near as much income as you, princess is only getting one-way hugs and kisses!!