I got a rear bumper dent/crack on my driver side from backing into a steel bollard last night.

What do you think the repair costs will be?

Is there no chance of concealing it without replacing the rear bumper completely?

Commercial bodyshops would replace the bumper rather than repair it. Cost? Almost impossible to estimate ( depends on demands in your area), but £350 to simply repaint a bumper would be a ball park starting figure. 
Options:- most of that damage can be brought back out  -on a DIY basis - via the application of reverse pressure and heat from a hair drier. This would be your cheapest route, and leaving residual damage to be dealt with during the repainting. 
Alternative:- buy a secondhand bumper and get that repainted. 
Tip:- enquire at small volume car sales sites in your locality. These chaps depend on getting bodywork rectification work done at reasonable cost, so always a good source for recommendations. 
Please let us know how you get on.

Regards,

Gareth. 

Wouldn't bother trying to repair the bumper. You can pick up a replacement Sportback bumper for around £175 plus £60 delivery. It doesn't need to be in colour, as 99% will undoubtedly be scratched, so it will need to be painted. If you can do that part, that will reduce costs. That means sanding it back, filling any scratches with plastic body filler and flatting, with various grit (start around P600). Paint wise, possibly £300-£500 ball-park for bumper alone.

Avoid, however tempting, new bumpers that are primed on Ebay. They will be non-genuine VAG parts, possible don't fit properly and will be inferior quality in terms of injection moulding. Also ensure the second hand bumper you have - has the pre-drilled parking sensor holes, as the mounting brackets are already added inside. 

Straight-forward to remove bumper. usually Torx screws and clips. Rear wheel arch liners will need to come off. If you don't have skills, then put it to a body-shop, but will be easily £1,500+. Silver is a nightmare to match even with the correct paint code, so a proper repair would blend into rear quarter too.

The other trim on the bumper on lower rear valance will need to be removed, mudguards, etc.

That offside rear light will need to be replaced too - to avoid water ingress.

Has any of the above been of any help to you Jeremiah? 
You seem to have posted your plea for help, but not returned since the day you posted. 
Regards,

Gareth.