Bare minimum would probably be a 180g because the width of the 150g is limiting - which is why I had to pass on a Fire Eel myself. However, there are other spiny eels that grow to about a foot in length so more then happy with going that route.
$30 is not a bad price since thay can eazily grow to 3' and as thick as your arm so a fully grown one would need a pretty large tank 200+ but at that size a 75 would last a few years