a new set of DPF parts will cost that much if you buy the parts from Audi
and as the sensor is seized in place then the parts will need replacing
now you can save money buy buying pattern parts from Eurocar parts and I'd also get a small independent garage to do the work as their labour rate will be considerably less than the dealers
and a small garage may also more motivated to try and remove the sensor so saving the cost of a new DPF as with enough effort you can usually remove a seized part ( drilling it out for example)
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