Why does this query become drastically slower when wrapped in a TVF?

I isolated the problem to one line in the query. Keeping in mind that the query is 160 lines long, and I'm including the relevant tables either way, if I disable this line from the SELECT clause:

COALESCE(V.Visits, 0) * COALESCE(ACS.AvgClickCost, GAAC.AvgAdCost, 0.00)

...the run time drops from 63 minutes to five seconds (inlining a CTE has made it slightly faster than the original seven-second query). Including either ACS.AvgClickCost or GAAC.AvgAdCost causes the run time to explode. What makes it especially odd is that these fields come from two subqueries which have, respectively, ten rows and three! They each run in zero seconds when run independently, and with the row counts being so short I would expect the join time to be trivial even using nested loops.

Any guesses as to why this seemingly-harmless calculation would throw off a TVF completely, while it runs very quickly as a stand-alone query?

I expect this has to do with parameter sniffing.

Some talk about the issues are here (and you can search SO for parameter sniffing.)

http://blogs.msdn.com/b/queryoptteam/archive/2006/03/31/565991.aspx