Ribosomal RNA amount in a Drosophila cell
The protocol you are using will not only leave the sample with rRNA but also non coding RNA.  

Many RNA protocols will separate mRNA by affinity of a carrier to the polyA tail.  This protocol references an older paper that estimates that only 5% of RNA is mRNA.  I'd be surprised if this ratio changed by more than 2-3 fold in drosophila. 

I assume that %age is by weight but it could be a densitometry measurement which is similarly interpreted.