First of all, I want to say how great it is that you're considering a double major! Double majoring is a great way to add breadth to your degree, and it lets you explore a lot of academic areas that would otherwise go untouched throughout the completion of your college degree.
As far as cost goes, it really depends on how time-intensive the majors are, as well as the school that you attend. If you're able to fulfill the unit/class requirements for both majors within four years, it shouldn't cost you any more than what would be expected at traditional four-year universities. However, if the double major postpones graduation by a year or two (because it takes more time to take all the classes necessary for both majors), it could end up costing you a lot more money than you expected. That being said, it all depends on what your career goals are. Double majoring in plant and animal sciences and paying the extra money could really be worth it for you! Or, it may be better to stick with one of them and graduate on time in four years.
Feel free to ask any additional questions!