Staying below 100mb ram is basically impossible I think. I made a small game ( 500kb in Construct 2) and when I build it with Crosswalk it still uses 120mb ram. So looks like there is a base usage which can not be avoided. But at least the ram usage evens out with larger files. My first game was 8mb and used 200mb ram.
I tested the 200mb ram game for a few ours on my Galaxy S3 (850mb free ram) and it didnt crash once. So the save number for your ram usage would be basically half of the devices ram (phone has 1g ram= app can be 512mb ram) But staying below 200mb ram is a save bet if you want to include the 512mb ram phones since there can always be mb ram spikes and you want to give your game some air and not run at the phones limit.
For IOS with Ejecta or Phonegap (when it gets viable in a week with IOS8 I have no experiences. But I have high hopes for Phonegap, so lets see).