Consider this not too uncommon scenario: Bob asks Rob to do a one-off job. Due to the nature of the job, giving a precise quotation is difficult, so Rob provides an estimate of $5,000 to get the job done. Bob says "ok, let's go ahead".
While doing the job, Rob incurs expenses he did not foresee. However, because he gave only an estimate, not a quotation, he is not worried about that and carries on; because it was only an estimate, he thinks that informing Bob of the extra expenses is unnecessary — he will accept what they factually are.
Finally, the job is all done and Rob sends Bob an invoice for $10,000.
To what extent can Bob dispute the invoice?
Different sources like this, this and this say that an overage about 10-20% would be reasonable, but what does common contract law say on how much Bob actually owes to Rob?