Zero :-)

We do not charge any upfront fees, setup fees or anything of the kind.  

What about registration fees?  Some of the sites I'm checking on charges $2.00-3.00/per registration.  Our company sometimes may have 20 attend a class, up to possibly 5,000 attending a conference.  For monthly budget purposes, this can be very unpredictable.

We do not charge that either.

Only flat monthly fee.

I think WA is up front with it's cost - a monthly fee depending on the size database you need.

As far as other associated costs - I don't think they will be to WA, but to other service providers.

The biggest "add-on" (not cost, but number of people that will need it) I can think of is email, as it is not included in the WA package so you may need to pick up a paid service elsewhere - unless you can get by on a freebie

The other "add-on" would be a domain name - but you pretty much buy that separately no matter where you go, to keep its portability - and you may buy your email there also

Depending on your needs, you might find yourself paying for additional 3rd party tools . . .
We have a public message board we will be keeping, We have our images hosted elsewhere to aid our portability and I don't see us moving them all and fixing all of the links once we get over here, etc.

But none of that gets paid to WA . . .

Thanks for the comment.

As we mention in our online help, free email by Google (on your own domain) is a good way to handle email.

http://help.wildapricot.com/display/DOC/Email+setup+and+WildApricot