From what I know of SNC-Lavalin's contracts, the firm does add a certain percentage to the contracts and mandates that it offers. I cannot say whether it does that for all contracts, but that is certainly the case with professional services. If I am asked to bid in order to do work or offer professional services to SNC-Lavalin, and I submit a $25,000 bid, SNC-Lavalin will top up that amount with its contract management expenses.
To my knowledge, SNC-Lavalin marks up all the contracts it offers and awards.