Cheap depends on your definition but the lowest end skylake CPUs for I7 are the 6700 and 6700k. K model starts with higher clock speed but also needs a cooler to be supplied, 6700 none k is lowered clocked but still 200mhz higher than your current one and high decent boost clock along with more threads. But both are over 300 bucks. You just can't buy a modern i7 under 300 atm save maybe is you found am mysterious used one.
edit: was wrong goofed none k woudl be same base clock.