Brief math gives me $4.22/mo:
2 15-watt lights @ 12 hours/day * 30 days/mo = 10.8 kWh/mo
1 80GPM filter est. 5W @ 24 hours/day * 30 days/mo = 3.6 kWh
1 50W heater @ 100% capacity 8 hours/day * 30 days/mo = 12 kWh
Total 26.4 kWh/mo @ estimated $0.16/kWh (after taxes/fees) = $4.22/mo.
This assumes:
1) You're running your lights 12 hours/day. If you use fake plants you probably don't need to do this.
2) I totally guessed at how often a heater is actually running and at what capacity.
For a worst case, if you're heater and lights were both on 24/7, you'd be talking $7.87/mo, obviously varying slightly based on how much your electricity actually costs.