The most cost-effective solution is a sissy bar bag, about $200 or a little more.
Mustang makes a really good looking hard leather-look trunk, but it's over $900 w/ hardware.
In between those price points are Givi and Shad top cases. On my V-Star 1300 I have a Shad 50L case, mounted on a Mutazu (sp?) rack I found on eBay. Total cost less than $500. Those cases can fairly easily be transferred to a different bike if needed.
2014 V-Star 1300 DeLuxe
2014 BMW R1200RT (Recalled/bought back)
1984 BMW R80RT (Sold)
2006 Suzuki Boulevard C50T (Sold)
Seven others, 6 sold, 1 wrecked