Hard to compare across countries. Ask around maybe, what kind of reputation do those schools have there? Are they academically high achieving and are they really value for money?
I wouldn't pay much for the bilingual school we have really, I see it only as an interim solution but academically I am not satisfied with it.
Initially I sent dd to a German school (partly because I already knew that the bilingual schools don't have a great reputation in my town) and did English with her at home in the afternoons. The German approach to schooling doesn't suit her at all, she was desperately unhappy in that school which I think was probably one of the better ones. This may not apply to your dc and the Italian approach though.
I had no option really other than to move her and sent her to a bilingual school because I knew a) we would get a place for year 2, being in the English stream and b)it would be a different approach based on the national curriculum so whole new teaching style. Also having looked around, I wasn't convinced the monolingual English internationals were great and I am not convinced that she would have been any happier at a different German school. Didn't feel I had a choice in it really.
When I applied, a teacher at the bilingual school drew me aside and advised me against it. In her opinion, the bilingual schools disadvantage English speakers because they are held back in German (this I found to be true) and yet are not moved along very well in English either (this I find is also true). She told me they work for German families whose dc end up with more English than they ordinarily would have but English speaking dc achieve less than in an English school and less than in a German school.
Socially, I have found it is a nicer environment for my dd so I don't regret the move, however long-term I don't see it as a solution for us.
In fact I see NO solution.
Maybe the Italian bilinguals, being expensive, are under more pressure to achieve academic results. If so, they may be worth the money you need to pay. I think in your shoes I would at least consider the preschool to give her a grounding in learning in both languages.
I wanted my dd to avoid having to attend English as a foreign language lessons since it is her mother tongue. It would have just been boring and frustrating for her I felt. (They also teach a weird pronounciation)