The easiest would be to say that if the federal tax rate would be added to the provincial rate, Qc would get an extra 50 billion $ a year. Now obviously some services such as immigration/border security/employment insurance/etc would be an extra cost to add to the budget. But a fair number of services are duplicated at the federal/provincial level such as healthcare administration /public health(while not everything pharma drug approval) justice, environment/agriculture and lots of others. Also expenses such as armed forces would be almost non-existent (at least initially) in a independent Quebec compared to current levels. (as an example, armed forces cost about 20 billion in Canada so Quebec funds about 20-25% based on population so about 4 billion in saving right there. Even if QC were to take 1 billion to build a Coast guard service its still 3 billion/year we aren't spending on used British subs)
but honestly i admit that the economic plan for a independent qc has never been really talked about in the mainstream media (thus not debated). But i know that some budget plan were made quite few years ago to show the economic upside of it)