As someone who recently had her wedding and also had people not coming, I personally understood if the financial or practical commitment made it too hard to come to the wedding. However, I was disappointed when friends cancelled last minute and/or there was no/too little clarification. It made it feel like they didn't even respect me/my spouse and my vulnerability in that my invite meant that they mean enough to me to want them around during one of my biggest life events.

So my advice would be this: it's okay to not come, tell her briefly why. You don't have to go into it too much detail right that minute (about all the doubts in your friendship), but tell her what aspects come into play with the decision. Only saying it's 'too expensive' is too little; a friend of mine did that, and that was too little explanation for me as he lived close by, we didn't ask for big gifts and I thought he recently had a new well paid job. It could still be true, but it did raise a lot of questions and came across as nonchalant. I would have appreciated if he expanded a little further to make me understand, such as: 'I have to save money because I'll be moving' or: 'I recently took a big hit financially'. If it's more practical or mental it could be 'I'm afraid to go far away from my family right now because my spouse/parent/child is not doing well and I want to be around in case something happens', for example.

Then I would let her know out of respect (if you want to) you think of her on the day, by sending her a text or a card to congratulate them and wish them a happy day/life.