100MB shouldn't really be a problem for a provider.  I agree with the previous post that a bare install with no modules (and no users/site sleeping), <30MB is probably true; however, I see memory utilization for several dozen DNN 4.8.4 sites on a single server ranging from 60 to 160MB with the majority at 100MB or above.  The highest are running Catalook stores, most have numerous modules installed and nearly all are using keepalive.aspx to keep them running.

If the provider is simply overloading their server or if additional charges for memory are being pending, you may want to consider moving. If you're happy with the service, then take Cathal's advice above and remove any unused modules from the module definitions page; you can always add them later as needed.  Also, make sure that you update any old modules to the latest or near-latest versions.   Even still, I wouldn't expect you to be able to get memory utilization down below 50-60MB especially if you are keeping it alive.  Now, if you are seeing slow load times when not using a keepalive, that means you don't have traffic because if you did, the visitors would keep it alive and you wouldn't have to... but that's a different problem altogether. :)