you will have to decide what type of build you plan to do.

I seen complete 8 speed groups that are listed for under 200 and 10 speed groups for under 300

brand new tiagra in the 300 range is a great value.

Finding a donor bike for part is likely the best way to get a complete group, just keep in mind the crank stem and bar sizes

you will likely need a complete set of housing and cables also.