I'll start with my BOM - I'll use the HomannDesigns site for pricing as is it's where I bought everything from, but a lot of places sell the same gear I'm sure.
Gecko G540 Stepper Drive + 48Vdc 7.3A Power Supply KIT = US$320
this includes The G540, a 7.3A Power Supply, and 4x DB9 (serial cable) connectors for your steppers.
Gecko G540 Stepper Drive + 48Vdc 7.3A Power Supply KIT
2 metre DB25 Male to DB25 Male printer port cable = US$7.70
Connects your G540 to your Computer, which must have a Parallel Port, or the means to install a PCI parallel card
2 metre DB25 Male to DB25 Male printer port cable
GC-02 DB-9 Stepper Breakout Board. (Four-Pack) = US$24
The gecko requires you to solder a resistor on the Stepper motors to set the correct current (SO3 steppers are 2Amps) these breakout boards have a trim pot, which allows you to just set the current without any need for soldering. Definitely worth it.
GC-02 DB-9 Stepper Breakout Board. (Four-Pack)
IEC Chassis Male Power Plug with Switch, Fuse Holder = US$9
Cable to the Power supply, not absolutely required, but makes it tidy and has a fuse in it
IEC Chassis Male Power Plug with Switch, Fuse Holder
2x 48VDC 20A Panel Mount Relay = US$8.50ea
These relays run from an input in the G540 and allow you to start and stop auxiliary equipment - I use one to start my VFD Spindle using Gcode Commands (M3 & M4), this means as soon as I push 'start' the motor spins up at the speed you instructed in the Gcode file and will turn itself off at the end. No more manually switching on and off, or manually setting speed of the spindle! - I have no experience with it but I believe the same setup could be done with a Makita/Dewalt router and a superPID.
48V 20A Panel Mount Relay
Aluminium Controller Enclosure = US$57
I didn't actually purchase this enclosure, I sourced one locally (should have ordered this one), but you will need some form of enclosure for the G540 and power supply, a common project is to make your own with the router from Acrylic
Aluminium Controller Enclosure
Small Latching Emergency Stop Switch = US$17
You could operate without this one, I did during setup, but the 1 time in 1000 when things turn to sh*t and you need to turn the machine off, a big red button is a lot easier to hit then a switch. Also kills the spindle, and wires straight into the G540
Small Latching Emergency Stop Switch
Up to you, it really depends where you want your control box to sit. I wanted mine close to the machine, so didn't extend my stepper cables.
I purchased some rolls of 22AWG cabling for extending my Homing switches and wiring from my VFD to my G540
MESA 5i25 IO PCI Card = US$89
This card is similar to the 'smoothstepper' for Mach3, but is designed only for LinuxCNC (smoothstepper is for Mach3 Only). The card allows a lot more inputs to and from the Router, has firmware specifically for the G540, and gives you real-time processing on the card itself, rather than from the PC. When running at high feeds on detailed work, with high-resolution signals (i.e lots of data, very high detail relief carvings are a good example) it is possible for the PC to lag and not keep up with the Program. Mach3 has a known history with it.
The card isn't absolutely required if you have a parallel port on your PC, I didn't think I'd be able to find a used PC with a Parallel port, so I ordered the 5i25, which has a Parallel Port on it. I ended up finding A computer with the PP and am still waiting for the 5i25 to arrive in the mail, but have been cutting basic jobs without issue on the PC's PP.
My research says this is worth it, and can be used in future when upgrading, albeit with LinuxCNC only.
MESA 5i25 IO Card
I was running my Shapeoko off a laptop, so I purchased a used desktop computer with the following specs: Intel I7 2.4GHZ, 8GB RAM, 240G SSD, 4GB graphics card for about US$300 including mouse and screen. I will upgrade to a touch screen in time as they appear to work very well with LinuxCNC.
TOTAL WITHOUT 5i25 or PC = $476
TOTAL INCLUDING 5i25 AND PC = $865
I would also recommend getting one of the options from Vectric Ltd. They have varying price points, but I dropped the cash on Aspire, which is their top of the range software, coming in at a hefty US$1995. If you intend to make money on what you are creating, that is a very small price to pay. The software is very easy to use, very powerful, and has amazing support. I have done a few relief carvings on my SO3, and the ones done with Aspire are faster, and have a better finish than anything else I have used. Also a lot easier than other software to just whip up a quick job or do a test. Highly Recommend.
Otherwise Fusion360 is great to use for more 'mechanical' type projects, it's free if you make less than $100,000 a year. It's more of a traditional 3D-modelling software, and has very powerful CAM functionality which can be used on anything from the SO3 up to Industrial machines. I either design in Solidworks, then import Into Fusion360 to do the CAM/toolpath creation, or do the whole lot in Fusion360
Hope this helps, I'm more than happy to answer any questions, I have used this forum a lot to learn and it's high time I gave a bit back.