I have been to Port Aventura for the last 2 years (stayed at one of the Port Aventura hotels) so do have some experience.
You dont say how long you are going for.
If you are going for a week then �400 is about �50 a day (including days you arrive and leave), which should be OK if you dont go too mad.
Your main expenses will be lunch, drinks (it is very hot even in June so you get through a lot of liquid), and maybe taxis to Port Aventura from Salou.
(Port Aventura is slightly on the edge of Salou and it can take about 20 minutes to walk from the centre of Salou to the Park. If your hotel is at the west end of Salou then more like 30 minutes or more).
Walking through Salou can be very unpleasant on a hot day but taxis seem to be very cheap.
There are a number of local excursions, the most popular being to Barcelona, but if you did that it would eat up your �50 just for the coach (you can get the train from Salou to Barcelona).
How many days do you plan to go to Port Aventura ?
I have to say this is an AMAZING theme park, one of the best I have ever been in, and you can spend days in there and not see everything.
While all the rides and shows are free the food and drink can be fairly epensive, like most theme parks.
There are loads of supermarkets selling cheap booze (and food) in Salou (including water) so maybe buy your drink BEFORE you go in the park and carry it around on your back in a rucksack.
Having said that, drink gets hot VERY quickly and there is nothing like a nice ice-cold glass of Coke while walking round a theme park on a hot day.
Here is map of Salou. It does not show Port Aventura, as it is off the map. It is above those two roundabouts close together near the top centre of the map (to the right of the railway line).
08:44 Sun 06th May 2007