The original budget was a guess. They didn't know how much the telescope would cost because the required technology didn't actually exist at the start of the bu

The original budget was a guess. They didn't know how much the telescope would cost because the required technology didn't actually exist at the start of the budgeting process 24 years ago. Now that technology does exist, and it turns out it's quite expensive, which drove most of the cost increases. However, since then the budget increases have generally paced with inflation. Source: your citation.