Very nice site.
For pricing, I would ask for MORE then the best spot out of the 4 gets per month and start from there. Then get rid of the worst out of the 4…likely you’re not going to sell all 4 to the same site and probably not all at the same time either. You can always do a little testing and see how much you can get. Put up a “test” ad.
Can work this problem backwards too…how much would I be happy with GUARANTEED per month from these spots divided by 4 = monthly cost.
Realistically though, if those spots are making +$400 a month each, you are going to have a hard time finding someone who is going to pay you that blind upfront.