We asked around in La Paz“ travel agencies for a taxi to Estanica Una. The best offer was 50US$ and we took it. Afterwards we asked other taxi drivers on the road an they told us 40US$ or less.
At the end it was a hard tour for the taxi and the driver unterstood why the others use 4x4.
But the cheapest way is to take a bus to Cohoni (3US$). Its not farther to walk than from Estancia Una and bus riding in Bolivia is as adventurous as climbing Illimani.
We used an agency AKA Pacha on Calle Sagarnaga. $75 each way on a micro bus. Some agencies wanted more, some less but I had no problems paying this amount as the drive to Pinaya is quite long and rough. It will take on average 3-4 hours.
We had one small issue and one big issue using this agency.
First, the small: we asked to be picked up at 11am. The driver didn't show up until 1pm. Again, small issue as this is Bolivia and everyone is on "Bolivia time - things happen when they happen".
Second, the big issue: On the drive up, the navigator (not the driver) decided he can take a short cut down into a valley...he said, "less turns". Well, after 45 minutes or so, we discovered that the road had been washed out. In fact, as we looked past this washout, we didn't even see a road on the other side!! We wasted something like 2 hours because of this "shortcut".