Not all enhancements are prohibitively expensive, although if your character is lacking in items, it's usually better to buy items than enhancements. On this particular card, yes, most of the enhancements would be fairly expensive. But making it a Heal 6 would only cost 30g, for instance.

Of course, once you enhance a card, there's a hefty premium for adding additional enhancements to it later on, and the more existing enhancements, the bigger the premium. But like anything else, you get what you pay for. Some cards have potential to be made crazy powerful with enhancements, provided you don't mind spending hundreds of gold to do so.

So far, the only time I've enhanced cards is when my character retires, since you can't take your money or items with you to the next character. So you sell all your items and enhance a couple of cards, and then retire. But I'd love to have a character that stuck around long enough to collect a bunch of enhancements, and see how much it changes things.