How hard depends on your solder and schematic reading skills. How expensive? components are cheap so just look at the price of an enclosure and material to build it on (breadboard or something). Those should be the bulk of the cost if you have all the tools. You can also check component prices online to see how they add up.
I'd say its doable but there is a good chance it won't work on your first try.
Note: I haven't read through the schemetic to check if there are any exotic components (digital) or other difficulties.
