Does that mean that if the covers are not of high resolution there well be differentiations and possible noise on dark areas or thy just wont be black enough and they will appear more greyish?
I use inexpensive HP coated 2-sided inkjet premium paper and have been very happy with it. And no, regular (plain) paper will not get the job done. Use the photo paper settings when printing, either matte or gloss settings according to the paper you use. Heavy-weight glossy photo papers will also look great, but tend to be too heavy for case inserts. The 2-sided brochure papers from HP seem to give the best all-around results. But the less costly premium inkjet paper is also quite good (matte finish).
Do you have any link of the paper you're using? The cheapest i can find from where i'm shopping is this
Also about the "gsm" indication, how does the number translate? Is it better to be thin so it can be smoother or deeper so it can aborb better the ink?
How much ink is used depends on what is being printed. The IP3300 is one of the cheapest printers to run there is.
So roughly, will printing 100 covers consume half my ink or Am i way out?