Look on RealOEM.com to identify the parts you need and the respective part numbers. You can then search for prices on those part numbers and estimate parts cost. Next is to add labor cost on top. If you're in areas like California or NYC, labor cost adds up really fast.

 

when i did my power steering refresh i did all 3 lines, pump and reservoir. at the time i did it, it was a bit cheaper but inflation jacked it all up to now almost $600 (pump doubled in price since 2018). my specific car is 2004 330ci with the LF-20 pump (some cars had different LF-XX pumps verify yours first on the tag) i did the work myself so no labor.

if you are mechanically inclined, the task is not too hard, jack front up to get under (ramps) get a drainpan and start removing hoses and remember how they are routed, remove belt, reservoir and pump, install new pump, reservoir, hoses, fill reservoir and start car, let it suck fluid in and top off again, then get car off ramps and cycle steering back and forth a few times then top off again. it will whine till filled. repeat till all air is out (will get foamy at first so let sit overnight and repeat next day)

again this is specific to my model. for example of what you will need to get for full refresh.

BMW Power Steering Return Hose - Rein 32416796390

BMW Power Steering Return Hose - Rein 32416796390

www.fcpeuro.com

BMW Power Steering Pressure Hose - Rein 32416796367

BMW Power Steering Pressure Hose - Rein 32416796367

www.fcpeuro.com

BMW Power Steering Suction Hose - Genuine BMW 32416750155

BMW Power Steering Suction Hose - Genuine BMW 32416750155

www.fcpeuro.com

BMW LF-20 Power Steering Pump - S-Tec 32416760034

BMW LF-20 Power Steering Pump - S-Tec 32416760034

www.fcpeuro.com

BMW Power Steering Reservoir - Rein 32416851217

BMW Power Steering Reservoir - Rein 32416851217

www.fcpeuro.com

$600

 

I just refreshed all my lines/res and got everything from FCP. You can save a bit more by getting the reservoir kit:

BMW Basic Power Steering Overhaul Kit - E46PSKIT1

BMW Basic Power Steering Overhaul Kit - E46PSKIT1

www.fcpeuro.com

If it were me, I'd pick up a used pump and save ~75% of the parts cost. As @das_autobahn says, the work itself isn't that difficult. Make sure you get the tool for the quick release lines if you do it.
Amazon:

Amazon.com

3d printable model I used:

BMW Quick Disconnect Tool by Zach | Download free STL model | Printables.com

 

I went with aftermarket from Rockauto. They are much cheaper and it is only the bearing that can go bad, I wish there was a rebuild kit available (like for Hondas) I would rebuilt it myself.

Notice that the OE pump has a plastic inlet while many aftermarket ones have metal.

 

I wish there was a rebuild kit available (like for Hondas) I would rebuilt it myself.

https://www.e46fanatics.com/threads/diy-luk-lf-30-steering-pump-rebuild.1294497/#post-18960054