Funding a class set of dry erase boards is not.
Don't get me wrong. If you are going to make an investment into your room, this is a good one. But, there are some alternatives that will save you some dough.
Shower Board: Home Depot and Lowe's both carry large pieces of shower board that work well with dry erase markers. They will even cut them to size for free. Home Depot is really good about making donations to classrooms so try asking the store manager. Even if you do have to buy them it will still only run you about $10.00-$12.00 (bonus tip: The Post Office has change of address packets available with a 10% of your purchase at Lowe's coupon inside. HD gladly honors Lowe's coupons too)
Target: The Dollar Spot currently has 2-sided dry erase boards that I've read good reviews on.
Dollar Store: You can find magnetic dry erase boards at Dollar Tree. They are designed for lockers and dorm doors. While the quality isn't great for this purpose, it does come with a small marker.
Page Protectors: Simply slide an 8.5 x 11" piece of cardboard and a piece of copy paper into a page protector and let the kids write on them with dry erase markers. They clean off pretty well and are low cost to replace.