If you promise that the job will be delivered on time... you are commiting professional suicide if you don't deliver on time.
Same is true with budgets. Unless the project brief has changed and you have alerted the client that there will be an additional fee, your project should be billed as agreed.
Which brings me to "word of mouth". This might sound surprising, but I rarely get business based on word of mouth. And here's why... my clients DON'T want me busy! The reason for this is when they are under the gun, they want me to be available to help them so they keep me as their secret weapon. It's like giving out the name of your favorite baby sitter... NOPE. You want your baby sitter available for only you.
But let me tell you this, and remember this well. If you screw up a job or are not treating your client fair, that's when "word of mouth" kicks in.
So... play fair. Quote a fair and reasonable price for your help and deliver on time all the time.