Cheapest way would be a V.3 Castle Monster Max and one of the chinese motors( feigo, Medusa).
For LiPos you can get the 5000mah 4s Zippy Flightmax ones from HobbyKing for about 100.00
a pack with the shipping from china. A charger will be another 150.00 - 300.00 depending on
if you go to a dual port unit or not.

So the cheapest conversion would be

V3 170.00
Feigo 7XL 70.00
2 lipos 200.00
Decent charger 150.00
Total 590.00

This still leaves out things like wiring parts, a motor heat sink ect.
You could be in 650.0 or so pretty easy. These are not cheap conversions,
the cost savings comes in the long run when you stop buying fuel, glow plugs
clutches, bearings ect. This of course assumes you don't let the smoke out
of your ESC, melt down a motor or jiffy pop one of your LiPos.

Yes there quiet,clean and more consistent run to run than a nitro car but they
carry there own set of headaches as well. Motor heat can be a real problem and
using them for brakes only amplifies this. Pinions fly off on a regular basis
and unless you want your car to sound like a SawzAll you will need to convert to a
plastic Spur gear. The MMM ESC are proving to be pretty reliable so you would
have to do something really boneheaded to kill one of those. LiPos still need to
be watched pretty carefully as life in an 1/8 scale buggy is pretty rough. Lastly
any weak spots in your drivetrain are gonna show up real fast so be prepared
to do some preventetive maintenance before you even get the brushless stuff
in.
These are not quite yet Plug and Play systems. There alot better than say two
years ago but you will still have quite a bit of sorting out to do once its up and
running. Gearing, ESC settings, Lipo cutoffs, motor cooling ect will all need
close attention for the first dozen or so runs. Now I don't want to sound
negative because I really love Brushless/Lipo 1/8 scale,but people really
need to go into these conversions a little better informed.