It can be tough trying to organize something romantic when you're on a budget, but I think the most important thing to keep the date romantic is to show her how much you care, and prove how well you know her.
A movie night could be a good way of doing that, proving you know which movies she's been looking forward to watching etc...
However, rather than make it an "ordinary" movie night, you could go all out by cooking her favorite meal.
It doesn't even need to be super complicated or expensive, as long as you know she'll love it. My girlfriend is usually won over with mac n cheese...
Also setting everything up in a romantic way so that she'll see just how much thought you've put into the date is a good idea.
Even a movie night can be made into a special evening with the help of some redecorating and mood lighting.
The key to making it special is catching her by surprize, doing the unexpected...
If you have access to a rooftop, then a rooftop date is always a winner (at least it seems to work on TV)
Another idea I like is having dinner out under the stars (so long as the weather holds up).
There's just something fun about leading your loved one out to an outdoor space, making them wonder where you're leading them to, and then surprizing them with a warm fire and a romantic meal.
If all else fails, two options that almost always add romance to an evening is bathtubs and alcohol.
You could even mix the too and enjoy some bubbles in the bath. If you're looking to save money, then cava or prosecco taste just as good as champagne, but are usually less expensive.
As you can see from the images above, candles are going to be a key expense though...