This is an interesting piece of meta on the possible cost of inception (the actual act of inception, not the cost of the movie ;D):
How Much Does Inception Cost? I'd take the initial section that speculates on payment with a grain of salt, since they're based off legal professions, and extraction is anything but legal. However, the parts afterward - particularly the speculation/theorising on how Saito would go about bribing officials to drop Cobb's case, and the cost/benefits of inception for Proclus Global - are worth a read.
The article's author also raises an interesting point when they ask: why Cobb isn't more proactive in his extractions? Why does he wait for a client? Why not just target a high profile company developing some innovative idea and then sell the details/plans to the highest bidder?
Now, to the best of my knowledge, there's nothing in canon that says Cobb and Arthur always wait for/look for clients. However, if we do accept the author's assumption that they (or maybe just Cobb, since Arthur tends to defer to Cobb's final judgement, for better or worse) only take client jobs, it lends credence to the idea that Cobb isn't used to criminal dreamshare. Actually, maybe he isn't used to being a criminal, period. With nothing to work off, perhaps he uses the field he used to work in - architecture - as something of a business model: wait for/compete for a client, get the contract, do the job.
In contrast, someone like Eames - well-versed in theft and various and sundry illegal activities - might very well use extraction in a much more proactive/aggressive manner, such as that outlined above.
Edited to add: I confess, this was written with fic in mind (since I no longer think of Inception without thinking of fic). So my speculating is made in the context of fics where Cobb and Mal were already criminals/quasi-criminals before Mal's suicide vs. fics with Cobb-the-academic vs. fics with Cobb-the-architect.