There are many sources of Tegus but a lot of sites will sell young pets for about $200 and some pet shops that stock reptiles might even be a little cheaper. You just need to look around where you live.
As far as monthly expenses, it depends what you feed. Live costs more so review the foods in the article. Plan on at least $25 per month. You also need to plan on monthly expenses like electricity (heating the cage), vitamins, bedding, and keep something available at all times in case your Tegu needs to see the veterinarian.