A single, large cylinder of helium holds about 240 standard cubic feet of gas. Value might be $50 to $100, give or take. To put it back into the cylinder, it needs to be compressed to about 2400 psi, so right there you need a 4 or 5 stage reciprocating machine that will cost tens of thousands of dollars if you can find one. Call
RIX, they would have a solution if anyone does. But I predict you'll hurl when you get the cost back.
Note that if you put it back into the cylinder, you won't get credit for it. Your helium supplier will likely be upset with you for contaminating his cylinder and may cut you off...
Realistically, if I had to do this myself, I'd first determine how much money I would save if I recovered the gas. If it's less than about $2000, I wouldn't bother. Next, I'd figure out how much gas I needed to store at anyone time. That would determine the size of the low pressure tank needed. A 200 gallon tank capable of holding a few hundred cubic feet of gas at low pressure costs about $1000 to $2000, so that would be the main expense. Optimizing this size would cut the costs.
Next, you'd need to evacuate the tank, so make sure it can handle full vacuum as well. You'd need to pull vacuum on it to remove all the air inside. If you know someone with a vacuum pump they could do it for you, then break vacuum only with helium. If you don't have access to a vacuum pump you'll need to buy one.
To get helium into the tank, I'd get a small, 2 stage air compressor and limit the discharge pressure on each stage to prevent it overheating. Helium has a ratio of specific heats of 1.67 where air is only 1.4, so it will get much hotter for any given pressure ratio. Probably want to keep the pressure ratio down to 3 to 1, give or take, so you might be able to get 100 to 120 psi out.
Note that none of this will purify the helium, and air will definitely get in. Probably not a big concern though given the gas is only being used in a balloon.