Heroku makes you pay to use specific features, why? how does this work with hosting solutions?

I'm currently working on developing a social network in rails. Right now I'm leaning towards using some sort of VPS/VPC (possibly AWS) but I'm still somewhat confused about a few things.

For example, in one of my prototypes that I hosted on Heroku I had a search feature using the sunspot gem however for this to work I had to pay Heroku for an add on(Websolr).

If I'm hosting on a VPS how would I go about implementing the same thing? Arn't there many hosting options that wont offer that specific websolr add on? Will the cost of running that search just be added into how much RAM i'm using?

Please feel free to critique my choice of going with a VPS/VPC. The reasons I'm doing this is because I would like to gain experience in managing my own server and also figured in the off chance that my app is a successthat it will be the best option as I will need a large amount of storage for uploaded files (users will be uploading music).