The issue is that you're probably going to grow algae. Your setup is designed for a higher plant mass than what you actually have. As for the cost of plants - plant prices fluctuate quite a lot depending on not only species, but varietal, not to mention availability for your area. Large crypts are expensive, and while Java Fern isn't usually too pricey, I've seen large ones go for quite a bit at auction. What you should do is get some easy stems that will grow floating (like hornwort or Myriophyllum mattogrossense) and/or one of the larger floaters like frogbit or something similar to start using the resources you're providing before the algae steps in. Both the easy stems and floaters should be inexpensive.