My need is a heavy use of routing. same as Google's Directions API. I'm looking for a viable alternative to abusing google and paying them too much.
how difficult is it to have my own server for that task?
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It's possible but performance isn't going to be anywhere near what you'd expect from google api on a small budget for an entire country. You should try it and report back :)
ArcGIS Server and Network Analyst is out of the question given the budget. So go with Postgresql, PostGIS and pgRouting.