Squeezebox

So a bit ago someone mentioned converting a Trek 5200 9 speed to 11.
I'm interested in changing my 5200 9 speed to lower gears. Compact crank 50/34, maybe 12-32 cassette, stick with 9 speed. I'm okay with everything else.
So I'm guessing new crank set, new cassette, new rear deraileur. Anything else? How much $$

Raiden

What is your current crank?

A new mtb rear derailer and cassette (and chain) is a fine idea, but I'd hold off on the crank until you're sure you need it. It's more than doubling your jobs cost for a relatively small change in your gearing.

If you're sourcing your own parts, I'd budget around $200-250 for the whole job, including tools. If you're having a shop do it, I'd say probably more like $400ish.

Andrew R Stewart 

I'd replace the chain and gear cables at the same time as the cassette and der. I also disagree with Raiden, 5 teeth off the front small ring is a very noticeable change. We do this change out a few times a year for customers. Figure around $150 to $200 for the der, cassette, chain and one cable installed by our shop dependent on the grade of the new parts. As two of the parts are consumables I wouldn't suggest high end models for the cassette and the chain. Andy.

Canker

RoadLink ? wolftoothcomponents.com
Any reason that wouldn't work? I know they only talk about 10 and 11 speed but I just figure that is because 9 is outdated.

Raiden

Keep in mind that the lower cage of your derailer (especially the shorter cage models that the part is designed for) can only handle a difference in chain length between the high and low gear of x number of links. If you put too wide of a cassette, then put a double (or worse yet, a triple) crank on your bike, with a short cage derailer and that adapter, the derailer will not be able to swing far enough and you will break things.

What that piece does well is lower a short cage read derailer away from the cassette so a big cassette has room to fit above it. I would assume performance across the small cogs would be iffy, since the derailer would be further way from the small cogs than it would be normally. Like cranking in your 'B' screw in all the way in for no reason. Then you could install something like an 11-36 or 11-40 cassette and (I think, maybe) still have enough rear derailer capacity for a narrow-range crank like a 46/36. But a 1x crank would probably be ideal (which make sense- take your old bike, add the adapter, a narrow/wide chain ring, and whatever cassette you want and bam, fresh drivetrain).

Al1943

The front derailleur hanger is a "braze-on" type and cannot be lowered enough for a big ring larger than 50 teeth - and even that may be questionable. I was able to move the FD on my wife's 5200 low enough for a 49 - 39 ring combination by grinding the hanger slot lower 2 - 3 mm. If you're not handy with a Dremel tool and round file don't try this. I think a 50 - 34 or 50 - 36 compact will probably work OK.