How toxic is baking polymer clay?
I am not an expert on this subject, by any means, but I can share the information that is available so that you can at least make a more informed decision yourself. Polymer clay, just like most other materials and chemicals, comes with an MSDS (Material Safety Data Sheet). This sheet lists all the potential hazards and risks when using the material. All the different types of polymer clay will have their own; however, they are basically very similar. The principal risk associated with polymer clay is that it can release hazardous fumes if it is overheated (hydrogen chloride vapor is one of the vapors). These vapors are irritants and should be avoided. Mainly, you need to make sure you do not overbake or burn the clays. The clay itself is considered non-toxic. 

You can see for yourself an example of the information provided in the MSDS for Kato polymer clay  at: Kato Polymer Clay MSDS.

There is an excellent article at The Artful Crafter, about the safety of polymer clays that addresses the use of kitchen ovens for curing polymer clay and can be accessed at The Artful Crafter - Polymer Clay Safety Tips. This article states that it is safe to use your kitchen oven to bake polymer clay and discusses some precautions if you are a heavy user of polymer clay. There are other similar articles on the web.

My clay oven is quite small so I have used my own kitchen oven on several occasions with no ill effects.