TLDR; If rates go up, you're locked in. If rates go down, get a loan estimate from a new lender (this can be done without an inquiry) and have them match or cancel and go with the new lender.
I think the cost you're referring to is the cost of points. If rates continue to go up, then this can get more expensive and your rate could be higher.
What is the actual cost to lock in the rate? I believe in most cases you'll need to pay for an appraisal upfront to lock in your rate. Some lenders will allow you to lock at no cost. The appraisal fee is usually refundable as long as the appraisal has not been performed.
Does your lender rate match? I would recommend locking now while shopping around for competitive rates. If you perform multiple apps within a week time frame (might be up to two weeks?), it won't have a negative impact on your score. If rates drop when you're closer to completion then get a competing rate from another lender or have your lender match it.
