What is the cheapest plane one can buy to fly above Mach 1 and be able to pull 8g plus?
Ignoring the various practical limitations that 99.9999991% of people would encounter.... 

The answer more has to do with what is available than what would be optimal. There are not a whole lot of air frames that fit such a criteria and there are even fewer that are for sale to/in the public hands, when talking about things that exist in the single digit realm for-sale quantity is a real factor. 

This McDonnell Douglass F-4 is currently listed for 3,950,000 USD and perhaps gives a good basis for what such a plane would cost. The F-4 carries 3335 gallons of Jet fuel, at my local field where fuel is a reasonable 4.50 a gallon its going to cost you 15000 USD to top off the tanks. Fuel burn depends on how you run the aircraft, if you are slamming it at super sonic speeds all the time your hourly burn rate is going to be high.   

Maintenance is harder to pin down. These jets were generally operated by the military so you are going to need to find a mechanic out in the world that is willing to deal with it. You may get lucky and find someone who has some military experience to do the work but if you cant you are going to need to hunt down all the maintenance manuals and related documentation for the aircraft which may be harder to come by than you think. On top of that you need to keep two J79 in working order that may already have a lot of time on them. 

If you want to hop to the other side of the pond design wise this MIG-29 is listed for 4.60 Million USD and will get you into the super sonic spectrum and allow you to pull some G's. 

So for ~4.5 Million and some non determinant maintenance costs, lets say conservatively 500,000 a year for fuel, oil, incidentals, maintenance, some new tires, avionics and other odds and ends you can have all the fun in the sky you want.