Depends where you're staying in Akumal. Are you at a resort (which other than Akumal Beach Resort, are not actually in Akumal), or are you right in Akumal? Which bay are you on - Half Moon, Akumal, Jade, Aventuras Akumal? There are several shops, all ok, so I'd pick based on convenience.
I've done a lot of diving in the Akumal area over many years, and I honestly wouldn't bother if I had to pay anymore than $30-35/tank. While any dive is a good dive, the ocean diving in this area, imo, isn't worth spending $40-50/tank, especially since it's really a no-frills type of diving. Carry your set up out to the boat, dive, go back to the beach. (I pay $85 or $90/2-tanks with a boutique shop in Cozumel, but they have steel tanks, long dives, beach club SI, towels, water, soft drinks, fruit and snacks, and they take care of my gear and store it, even the wetsuits)
In the Akumal area, diving is done as 3, single tank trips with your SI back at the beach/dive shop. The shop I've used for most of my ocean diving has a 4 dive pkg for $120, 6 for $160, 10 for $250. Less than 4 tanks and you'll pay $35 or $36/tank. These are prices for single tank dives. I stay on Half Moon Bay so I usually use Akumal Dive Adventures since they're on that bay and I've always had a great time with them.