How much to spend on yixing

David, not sure why mildew is forming perhaps you left some tea in or other organic material. All it takes is 1 mistake such as leaving a little water, tea, other organic material and the mildew can start to grow. If not cleaned properly the mildew that is stuck in the pores will eventually grow back as well.
So what i would do is boil the entire pot for an extended period like 45 minutes -1 hour this will destroy everything living in there and in the pores. If for some reason that does not work soak it in some sort of alcohol like gin for 24 hours then boil it a few times to get the alcohol out. That should clear up any issues.

Thank you for all the advice!

this site has lovely, not to dear yixings.
http://www.teahouseoolong.eu/nl/index.html (tax applicable to outside EU).

I bought a Yixing via a local action house, I was lucky, my lot was valued at 100 euros, but as there were no buyers on that day and not much interest, my bid of 40 euros was accepted.
I am pretty pleased with it, and think it’ll do nicely for a first time buy. Will eventually look at others as well, but not now.

approx. 45 USD would be just fine.

So many of the Yixing pots sold now are a softer clay. The good Clay is gone unless you’re buying vintage. I think Jian Shui is a better buy these days.