That is cute about the betta and the cories!
For an upgrade, it could be as low as $14 to go from a 5g to a 10g. Alot of filters that are rated for a 5g tank will go past 10g, which would decrease the (possibly) strong current. Heaters rated as low as 50W would work fine for a 10g. You could add more gravel later if need be.
Also, Petsmart has 10g packages available for $45 for members that includes a heater. That is the best deal I have seen for a new package.
FireOasis - 3g
Zachary the Halfmoon Plakat
CichlidHaven - 55g (under construction!)
0.0.7 Black Widow Tetra
1.0.1 Convict Cichlid
0.1.0 Firemouth Cichlid
HermitBase - 10g
Zephus & Monoculus the Purple Pinchers (Coenobita clypeatus)
x1 Convict Cichlid Pair (actively breeding)