The base amount of XP for the Weather Station is 1000 XP for a total of 7125 money. If you are going to try to convert money into XP, you should probably use the Volleyball Net. That has a base XP of 500 for 3375 money, so you'll need to by 2 Volleyball Nets for every Weather station. The absolute best item is the 25$ shrub, but that takes for ever to earn any substantial XP.
I actually wrote a program to calculate the best items for converting money to XP. The 25$ shrub is always the best regardless of your bonus setting. There are a couple items that are more efficient than the Volleyball Net depending on your bonus (the 100$ tree and the 135$ Pylon), but the XP earned is low enough that I think the Volleyball Net is the way to go.