Hi Angela,
The ticking noise could be about half a dozen things depending on whether the noise is coming from the top, bottom or side of the ending. The bad news is, whichever problem it is, it could cost a fairly large percentage of the value of your car. So let’s focus on replacing it.
A small automatic diesel will be cheap to run. For example, a Ford Fiesta 1.4 Duratorq TDCi would give an overall mpg of 62 mpg. However, small cars, especially automatics are never cheap to buy. It’s catch 22. Your cheapest option will be a used mid-size hatchback that also delivers good mpg. Consider another Astra, a Focus or 307.