Errm. Lvl13 amp has 20 drops. 1 claim per amp is exactly what you're supposed to get indoor, 5% hitrate. What's killing you there is the fluctuation of respective find, both in terms of the class of claim (significant, plentiful etc) and then in terms of fluctuation of respective class within own limits. For reqular outdoor mining the loot events are many enough and claimsizes small enough even with L13 to spread that risk to a manageable level (ignoring MU). But indoor, even at 100k turnover, you're only looking at 280ish finds. That's nothing when possible claim classes alone are (iirc) 10, not even mentioning what fluctuation brings something like enormous (2 to 4,99k), rich (5k to iirc 29,99k) etcetc And then there's the tax, presuming three turnovers at perfect breakeven, it means 0.95*0,95*0,95, which would be a loss of almost 15% tt to tax alone in barely 800ish claims. Then comes the MU.
It's okay to gamble if so you feel sometimes, but know your risks properly. 5% is not bad or good, is the normal indoor.