the thing is with chart plotters is they are seldom much cheaper than combo units . and more often than not makes sense to just get a combo unit for resale . or back up reasons
in that budget... you got a few options "pretty much all 7" though"
HDS7t.... + ethernet cable. pair it up with your existing hds8.. . runs fine. far better than the hds8 your currently running. but at least its all language you understand "and kinda easier"
garmin echomap 70s (or 751xs.. if you want a bit more features). the 70s will run far more mapping options than the lowrance. far better gps technology. easier to use and cheaper
simrad nss7. "similar reasons to the hds7t.. just not quite as fast. or as big. or as many toys". but cheaper
all of them will fit your budget.
moose