You can buy a PAYG dongle from Orange or Vodafone in Salou Old Town. Fingers crossed that they have one in stock and the assistant can speak English
All depends what you are doing as to how much you put on it. Link it to an online a/c and you can top it up from your browser.
If you have an unlocked android or iPhone a better idea is to buy a Lebara SIM from one of the many Locturio's in Salou, pop that in your phone and link your laptop to it as a WiFi hotspot or via USB cable (known as tethering). Data via Lebara is up to 1mb per day free and 15 cents per Mb thereafter. Good for a few emails or social networking but not brilliant if you are going to be downloading anything big

You could get a PAYG UK T-mobile internet dongle (about £20.00) and get a Euro Booster then when your holiday in Salou is over you could use it back in the UK as well.
I don't think the cost is too bad but as The AV Kid said, it depends on what you need it for.
We use ours quite a bit, its a lot cheaper and convenient if you are away from home and can't find free wifi.

I've got a 3 mobile dongle, will that work?

Hi Andy, I don't think 3 mobile have anything like T-mobile's Euro booster but check their website to be sure, since first replying I've had a look on ebay and there are T-mobile dongles for about £15.00 or cheaper, the best option may turn out to be getting your dongle unlocked, have a look on ebay or google.

The Lebara SIM will not work in an unlocked dongle. You will still need to go to Voda or Orange and get a data SIM.
If you cannot go the Android/iPhone Lebara route then Lee's suggestion is a good one and will almost certainly be cheaper than any option of unlock/Voda/Orange in Spain. The Spanish PTT's are still a lot dearer than UK/US prices.
I would contact 3 and ask them for a data roaming quote. If this is expensive get a T-Mobile dongle with a euro booster.

"down town" are plenty of bars offer free wifi

I don't realy want to be carrying a laptop out with me, the idea was that we could use the internet from our room or sat outside at the hotel, usually hotel WiFi is expensive and the range is very limited, the idea of the dongle would give a little more flexibility, I can do free WiFi from my phone but as it's only a Blackberry the screen is not the best for internet browsing

You should be able to link to your Blackberry via wifi and use that to access internet..as mentioned above, it is called tethering.

I can get WiFi from my laptop, that's not the reason for my question, I want a reliable internet dongle so we can use the laptop without being limited to a very small area of the hotel and paying OTT for the privelidge.

Just google BlackberryForum, and then search 'How to use Blackberry as a modem'. Save us both a lot of time and effort trying to explain here

Perhaps instead of being rude you could understand that not everyone may be as techy as you, by linking the laptop to my blackberry I was thinking you were suggesting I use the phones WiFi which obviously is not what I wanted.

Rude??? Wow.....