What kind of returns could I expect, say I had 10k investments and started harvesting, how many xems could I get?
You'd earn ~400 XEM a day plus harvesting fees if you invest 33 BTC (at today's price) in a supernode.
33 BTC for a supernode? Is there too much money?
It depends on the price of XEM and BTC, in January 2018 the price was approx 380 BTC (~$3M), but that was at the ATH. Think of it as an investment, you receive around 3.65% ROI. At the low prices currently it is an excellent long term investment.
See this thread for some idea:
https://forum.nem.io/t/supernode-payout-only-2/10312