Barrows items are dropped, on average, 1:15 iirc. The sizes of these items vary from 50K to 1.7M in the present. A lot of them cheap runescape gold are at 1M, and even below, few are above 1M. Also, let's say 1M per 15 minutes - this is a good estimate for runes as well as other things in addition to. Barrows is very affordable and you end up with lots of death and blood runes leftover...