Hi,
That is very difficult, because the price for i3-2105 for example is 164.17 USD. So if you need i5 then you will need to at least $179.99.
Maybe you should try to look for used i5 cpu then you can maybe get it for 100$ or less.
Note: I am assuming your MB has LGA1155 socket.
Note: These prices is according to pcpartpicker, which is from websites like newegg, amazon, and other stores.
Regards,