As far as cost, you can buy and pay to install a good Sound board Transducer (SBT) such as a K&K Big Shot Internal for well under $100 (McIntyre's are much more expensive). Under saddle Transducers (UST) will be much more expensive to buy and more expnsive to install. I would figure at least $150 - $200 to purchase and install PROPERLY (getting the string to string balance can take a LOT of time/money). With either, you must use a preamp (some of the UST's come with an internal preamp, which does add noticeable weight to the ukulele).
Here's the K&K link:
http://www.kksound.com/bigshotinternal.html
In the for what it's worth category as you will get a bunch of differing opinions on pickups, here's my recommendation:
I much prefer soundboard transducers (SBT) to undersaddle transducers (UST) on ukuleles. The smaller body of a ukulele works very nicely with SBT's, and the sound reproduction is very faithful. On my 4 ukuleles, I have 2 different pickups. Both of my Kanileas have McIntyre Feather pickups, my Mainland and Kala both have K&K Big Shot Internal. I like the McIntyres slightly better, this, however, could be a function of them ( the McIntyre Feathers) being installed in much better (and much more expensive) ukuleles. I find I have to EQ the K&K's more but, again, that is probably the ukuleles themselves. You MUST use a preamp with any of these pickups. I use a Fishman Aura Spectrum (I am mainly a guitar player). When I use Aura with the ukuleles, I am using it (the Aura) only as a preamp, no imaging.
I should mention that my Kala had a UST in it (LR Baggs Element with Mi-Si preamp). I pulled the pickup and preamp out almost immediately. UST's (IMO) in ukuleles just don't sound like the ukulele, they sound like a BAD higher pitched cheap guitar. I use UST's on all of my guitars so I like the pickups in bigger bodied instruments. Another MAJOR problem with UST's is string to string balance. The balance on my Kala was horrendous, which didn't matter, as I was taking it out anyway. The saddle slot and bottom of the saddle MUST be perfectly flat to have proper string balance (Something you don't have to fight with a SBT).
Jimmy