If you are concerned simply from the monetary angle of things, I just upgraded to mechanical disc brakes on one of my rides and the grand total after all was said and done was just under $500.
I tried to stay mid-range on the prices of things (for the most part), and some trail and error caused some extra expenses, but here is my rough breakdown if you want it:
2007 Avid BB7 Mechanical x 2 – $120
SRAM Flak Jacket cables x 2 – $60
Mavic Crossride wheelset – $225
Brake Mount Adapter – 40
Assorted Cable mounts – $20
I kept my existing XT brake levers. I did the job myself, not tough, but had to buy the lockring cassete remover, housing and cable cutters, and chain whip (they will run you anywhere from $40-$70 or so total depending on the design and brand of tools).
The cables I bought were to keep the cables internally housed from lever to caliper. I really like riding in the cold of winter – no crowds and messy to frozen conditions – so switching to disc and keeping the cables shielded from the elements were highest on the list. I ended up using most of the second cable set purchased (a $30 pair is meant to set-up both brakes). I chalk that up to a piss poor Sette/Torx brand cable and housing cutters (buy Park tools), my inexperience, and a poor design in regard to the part of the cable system designed to use your existing cable stops – the part where your cables are likely bare and exposed now under or on top of your top tube. The brake cable kit includes some adapters and red/pink tubing to internally house the usually bare parts of the cable between the cable stops. I didn’t like how it looked, nor the design of it. I scrapped that after I initially installed it (thus the second set of cables) and just ran cable along the frame the whole way for the rear brake. Much better looking, and easier to install.
The bike is a Fisher Sugar 2 FS, and the mounts on the rear are not directly compatible with the mounts of the brakes themselves. I chose to ebay it for the adapter that then allowed me to mount the brakes. You’ll likely save $40 there by not needing an adapter. You can still find older, new BB7’s (arguably the best mechanicals out there – I have a set on my hardtail 29er, which fueled choosing them for this project). I think I have seen them for under $50 each, and even at $40 or slightly below for older models. You can also spend $10 for just normal cable and housing, and save around $100 on a wheelset (trying to research and pick out a good wheelset that won’t cost you $400-800 dollars starts to suck quickly. If I would’ve spent more it would have been in this area, but I ended up with an ok wheelset for the price). A realistic grand total might be $250 or so for lower end with still new parts (with a cheap wheelset being the biggest place to save, but also the place to lose the most quality).
Except for one tool, I made my purchases entirely online. I swear, the LBS’s here just don’t get it when a customer walks in the door. It is like they don’t want to help you, nor stay in business so I oblige and take my money to PricePoint, JensonUSA, etc. Most shops will likely install these for free (or for a small charge) if you buy your stuff there, but I am trying to at least seem like I know how to work on my bike, even if it is something as simple as mechanical brakes. If I hear another half interested bike shop employee tell me that ‘we don’t carry that, but we can order anything’ while playing with their cellphone and not making eye contact I may never buy something from an LBS again.