UKMatt,

Our PB quoted us with plain white plaster too but gave us a price sheet with the add on costs. YOur PB may have that accessible for you. Your location, compaired to mine may differ as well. Here is our breakdown.
our pool is 675 sf.
Add colored quartz to white plaster: $1635
Sunstone upgraded colored plaster: $3161
Upgraded aggregate finish: $4578.
Our PB does not use Pebble Tech but another brand. I do not know how that brand compares in price to Pebtech.
Hope that helps some.
Laura

My PB has quoted me $2900 to upgrade from white plaster to StoneScapes Mini, which is comparable to Pebble Sheen. My pool is about the same size as yours, but I think geography plays a part in the price. I'm in the DFW area, so I'm probably going to get a better price than someone in California. My PB said that Pebble Tec would cost about $1000 more. The warranty on StoneScapes is only 1 year (although I've never heard anything to suggest that it won't last for 10), but if the warranty on Pebble Tech is significantly better, I might still consider it for myself. My only other concern with StoneScapes is lack of pictures showing examples of colors in actual pools. I'm hoping to be at the point in my build later this week when the pool gets plastered, so I'll post some pictures of the StoneScapes color I choose if it would be helpful.

In April of '06, a self-described pool plasterer on this site (msg no longer available for viewing - I tried to link it) listed these proportions:
Marcite...over $3/sqft.
Diamond Brite...$3.50/
Pebble Tec...$4.00/
Pebble Sheen...$4.50/

While prices may have gone up since then, IF they've gone up proportionately among all products, then you can use your price for plain plaster and get a ballpark figure for Pebble products...looks like Pebble Tec was running less than 33% more than plain plaster, Pebble Sheen running less than 50% more.

We just did this 3 days ago. Our pool is 14x32, going from 3' to 5'9" (terrible rock, would have cost another fortune to hit 6 feet) with a 6 foot spa. The prices my PB showed me were the same for both PebbleTec and PebbleSheen (he was surprised to see that, he said PebbleSheen has always been more money). Depending on what tier you pick, the price varies. We went with a Tier 2 (I believe) PebbleSheen and his direct cost is $4.40/sf. We subtracted out what he was charging for DB, added the $600 for the spa and he added some money in for him, of course. In the end, he's charging us $3500 to upgrade to PebbleSheen. We probably could have gotten him to come down more, but he's already included a lot of work at no additional cost, so I didn't push too much.

Our pool is slightly larger, and our PebbleTec black pearl came out looking stunning. It actually makes the water sparkle. It cost us about $4,500. in full. Love the look and feel.

My guess is $3000 for Stonescape Mini.
$4500 for PebbleSheen.
(standard colors)

Your PB may give you a credit for the plaster after the switch out but probably not since there isn't much profit in that in the first place.