I saw that some Weapon Special Abilities cost money instead of the enhancement bonus price (for example, an Impervious enhancement costs +3000 g, while a Flaming enhancement costs +1 bonus).
Can I add Transformative to a +5 Vorpal Longsword?
Or Transformative counts as a +4/5 bonus?
Abilities like Transformative that have a fixed monetary cost do not have an Equivalent Enhancement Bonus, so they do not count towards the +10 limit. You can add as many of them as you can afford.