Sometimes, you need to change the unit cost of a job, but just for one job. Your next jobs for this client will have to be invoiced at your 'normal' rate (the one you defined at the client level, under Clients > Prices tab of the client).
There is a way of managing this in LSP.expert.
If you don't need any details on your invoice (no matches, no wordcount), you can use the 'Fixed price' field in the job to define the one-off price of your job.
If you do need details on your invoice:
You create the job normally, and the system will automatically calculate the price based on the rate you defined at the client level.
You change the unit cost in the job by clicking on Edit in the Billing panel.
This will only apply to the job in question. It won't update the price of your service at the client level.