for most committed relationships flowers and a necklace go a long way. try and think of something unique she really likes if you want to make it more personal. it shouldnt be that hard to think of something.
give her an orgasm, and a good bouquet. none of that 12 rose bullshit, I'm talking a proper arrangement with an orchid centerpiece and some dahlias to compliment.
I've heard the absolute cheapest thing they have is around 10k...but hell if that's an affordable valentines day gift more power to ya. Or you could just make her dinner/take her out to dinner, either way
if she's you're into/dating/whatever, find a way to get onto her pinterest. you'll never run out of gift ideas and she'll always be satisfied and wonder how you always know what she wants