I doubt we did, allow me to remind you the paying dlcs were on the SP part of the game, not the MP one. Actually, you could say the MP was piggy-backing on the SP, since all the lore, initial game mechanics and aesthetic development was paid by the SP budget. Both had an initial number of dlc planned, paying for SP and free for MP because they didn't want to divide the MP community. The content for the MP was extended thanks to its microtransactions success. The budget for each game (I'm going to consider them as two separate games, for ease of explication) are not transferable to one another, even though they are added on one another in the final count. Specific expanses and possible profits for each are kept on separate columns, because you need to be able to justify to your investors how the money planned for one of the games was spent. It is similar for profits. By default here, both games start with the same amount of profits, since they come together, the difference comes with micro-transactions. DLCs for the SP, in-game credits for the MP. Such benefice are also kept on different columns because investors wants to know what sold better (and eventually why).
If the future MEAMP and MEASP were developped and sold completely separately, the money from the MP wouldn't pay for anything in SP, just as ME3MP didn't pay anything for ME3SP. The SP team probably had to cut in their profit for the Extended Cut, actually. That wouldn't be a bad thing for the MP. It wouldn't change a thing to what was done for ME3. MEAMP would go on merrily piggy-backing MEASP.