We have a test Amazon server that we have run PxPlus on. About the only wrinkle was to make sure the firewall allowed the inbound connections.
As for estimating cost, that totally depends on your application.
Now as for my personal opinion, while the whole world seems to want to move the Cloud, personally I warn clients that by moving their application off premise they become reliant on good, reliable, high speed internet access. When the host server is in-house, their in-building network wiring is generally 100MB to 1000MB. Once they move to the cloud they most likely will not have the same network speed.
Also, just to reinforce my position, here in Canada last Friday one of the major Internet carriers went down and took out around 25% of the country for 1-2 days (some parts are still having trouble). Could their business run without access to ANY of their files/applications for 1-2 days?
To me, given a choice, I would always chose to host an application in-house, provide in-bound controlled access, and use the cloud for backup and potentially emergency hosting.
A little up-front planning, good automated backups, and a bit of redundant hardware generally will provide a high degree of system availability.