They vary a lot. But that isnt cheap for a plain bumper, on top of that you need
Grill - £50 ish for either second hand or a non gen new
Fogs - £25 for new non gen
Fog Grills - £50 i think from Audi / TPS only.
I paid £50 for mine, was a new bumper, that a bodyshop had painted, but either never fitted, or was the wrong one for the car. Still needed reprepping, as it was scuffed from storage.
Fronts are a lot easier to find than rears! As you can use 3 door, SB or Cab. Rears have to be the right one, ie 3 door if you have 3 door.
I eventually found a rear 2009 for mine (rear year doesnt matter as long as its 8P) Paid £70 with diffuser, but needed a fair bit of work. Had PDC holes, one big dent, and some small ones, all on the corner curve, so not easy to repair. Though from the pic was hard to see where they were.
Rears tend to either be dirt cheap, but wrong model (for me) was loads of SB and Cabs on ebay, or missing the diffuser section. Which you will struggle to get used. And new for the honeycomb style ones are circa £200.