A dremel to build a brass breech isnt exactly machining hardware.
Anyway, the cheapest possible mods are probably rebarrels for single-shot blasters and spring spacers. Buying replacement springs isnt too expensive, but is usually as much as/more than the material for a brass breech.
It seems to me that springer mods can be cheaper than flywheel mods, but that doing a full workover on a springer is usually going to cost more/take more time than a full workover on a flywheeler.
If youre starting from nothing and dont have the cash to build up a stash of mags, a magnum superdrum or Zuru turbo advance are probably good blasters to start back up with.
I disagree a stryfe or rapidstrike or other flywheel with 18650 or 26650 or 14500 are by far easier than doing any springer mods... and the rewards are instant... pop a pair of 26650 into a rapidstrike with a inch thick spacer or mod the tray and dump a 2s or 3s pack of choice in the rapidstrike night and day better... and from there you can jump to a rewire, motors, cages, flywheels, and li-po mods, motor controllers.
My first mod was to drop a pair of 14500 IMR into a stryfe with a dummy battery i made out of wood glue paper and a copper wire in fact i made 2 dummies one 2 batteries long one 1 battery long. My next step was a semi rewire where i removed the thermal fuse and all the extra safety switches. Sometime after that i added a voltage monitor and hooked it into the jam door switch. My next step is upgraded wiring and perhaps flywheel and cage swap there is an aluminum set i was eyeing.