Around $400-$600. More if you need to do in-depth troubleshooting.
The 3D printer OP uses is a Monoprice; it looks like the delta mini (https://www.monoprice.com/product?p_id=21666). It costs $160.
The circuit boards and the solder paste mask are made by another company and delivered by mail. It's been a while since I looked up costs, but it's quite cheap these days, and the watch's circuit board is small, so I can comfortably say it cost under $100 to get the mask and a few copies of the board.
The soldering could be done with a $200 chinese reflow oven, like the T962 (https://www.ebay.com/sch/i.html?_nkw=t962), or with a $20 toaster oven, an oven thermometer, and some careful timing. (Some people do their surface-mount soldering with just a frying pan and their kitchen stove!)
The rest is just basic hand tools -- screwdriver, file, sandpaper, tweezers, maybe a magnifier. Maybe a $5 Arduino clone to help with debugging and programming, and a $35 T12 soldering tip+controller for doing soldering test stuff and "I messed up" wires.
If the first try doesn't work, and it's not obvious why, you'll to spend more money.
If fixing the problem requires changing the board board then you need to order new ones, and the mask to solder them.
DC voltage levels, current consumption, and electrical continuity can be checked with a $20 multimeter. The presence, frequency, and integrity of signals can be checked with a good used analog oscilloscope costing $100-$200. Some things are easier to check with a new, basic digital oscilloscope costing $300-$400.
If the electronics are good but there's something wrong with the software, you'll need something to read the signals the chips are exchanging with each other. Basic, low-speed (less than 1 MHz or so) signals can be read and generated with the Arduino board, or a $30 Bus Pirate. If your project uses higher-speed signals, you'll need a logic analyzer. A good USB model is the Saleae Logic 8, which costs $400.
(There is overlap in the capabilities of digital multimeters and logic analyzers, so you can sometimes just buy one and use it for both purposes.)