Camilla and Charles did not have a huge wedding, and it was a second wedding for both. Her wearing a tiara to the wedding would have looked bizarre to say the least. It was nothing to do with her being a divorcee. If she wanted to wear a tiara she could very well have worn her family one but chose to wear none. And since her wedding she has worn numerous pieces.
Harry is only 5th??? When Edward married he was 7th in line for the throne. He was the third son of the queen. And at the time there was little plan for Sophie or Edward to be working royals. Since that changed, since they became working royals, she has been given access to a family tiara, the five aquamarine tiara.
Harry may be a grandson right now, but he is the second son of the future king. And until George is an adult and taking on full time royal duties, that makes him a very high position as senior royal. He will be performing major royal duties for many decades to come as will his wife.
The royals are also very more conscious about finances and the situation. Buying a brand new tiara when they have entire vaults, would not be a smart choice at this time. Now if they crafted one from say one of the necklaces, or at the very least bought one from auction, the cost would be explainable.