Both quotes sound very expensive to me - there's a big difference between plastering garden wall as opposed to internal walls.
Couple of questions - why do you want your garden walls plastered in the first place?
Would you not consider just painting them yourself with wall paint?
I had my garden walls plastered in a really rough "mediterranean style" plaster, cost a fraction of the price you would pay for perfect plastering (no skimming etc) and then just painted them myself.
Don't want to sound boastful here, but my walls were the envy of the neighbourhood! Add to that some lovely exotic shrubs, grasses and a rockery etc etc..