I have an idea...
This, of course, depends on how well you know her, but if you know something she'd like to try (in bed?) help her realize that fantasy.
Or do this romantic stuff with a night in, candles (cheap) and some nice food (can be done cheaply) and if your budget allows a bottle of champagne.
Give her a bath (if you have a tub)
and be really sensual about it.
Massages don't cost anything, if you have some oil.
(if you don't, stuff like almond/walnut oil, grapeseed oil and avacado oil are good, and if you add a little lavender it gets even more relaxing.)
But basically, you can go for all these sensual things that every (?) woman dreams about, and it won't cost you much, if anything at all.
Breakfast on the bed is a winner..
(Let her know you are focusing on her the entire day (that means don't focus on your own pleasure, but hers), you don't need to say it explicitly, you can let her find out through experience...)