1928 is a fairly common date for Walking Liberty halves. They were only minted at San Francisco that year so all of them should have an S mint mark at roughly the 8:00 position on the back.
Numismedia lists the following approximate retail values as of 03/2010:
Very worn condition - $8
Moderately worn - $16
Slightly worn - $69
Almost no wear - $489
Uncirculated - $1,020 to $24,700 depending on quality