Cheap , you won't go wrong with a CP1 CO2 pistol .
Reasonable money you could look at Rohm pistols , both in CO2 and PCP.
Excellent value and accuracy .
I picked up a nice Rohm PCP with 3 mags and single shot adapter for £200
SMK pp700 are available for around £150, don't be worried about it being SMK.
£300 and your choice widens .
Things to consider ;
Single shot or multi shot ,
Type and size of grip
What type of shooting are you going to use it for ? Paper punching ? Plinking ? What distance ?
All of these things need to be considered .