Ideally the expectation per week is 10 hrs. Minimum GPU machine costs $0.4/hr on paperspace, so 40 * 0.4 = $16 per month. Add storage costs of $5 minimum and public IP (makes life easier) $3 = $24 per month.
If you run lots of models and want to participate in competition, go ahead with $7 storage and it comes around to $26pm.
Usually we might end up running for few hrs more than that and add some miscellaneous costs (setup etc.), so you can expect approximately $30 pm minimum bill for fairly conservative usage.