Foam Board and Roadbed

Without knowing which foam board you are talking about it is hard to tell (you may want to post a link on Amazon to what you are referring to). Generally, the so called pink or blue foam insulation board can work well as roadbed. If the cheap stuff is the white styrofoam board kind of thing, it won't work well. 

As far as toxicity, the pink or blue board is not toxic, it doesn't put off toxic fumes unless you decide to burn it for some reason.  Given you likely will be painting it or otherwise covering it, even if it were leaching fumes that would likely encapsulate it, but again I don't think you need to worry about that.