Where can i get the cheapest tobacco in rotterdam?

In an ALDI supermarket.They have packs for 2.50 euro
In europe, you'll pay a fix price for tobacco in every country (but these prices differ from 1 country to another), so unless you wanna buy contraband tobacco (for which I can't help you), the prices will be the same wherever you go in the Netherlands
In any tobaccoshop. It's the cheapest to buy the 200 grams cans, ask for the cheapest brands. Like 'Jacobs' (which is cheap but still good tasting). You pay about 15 euro's for that. You can buy cheaper, but beware of the taste.
(to compare: a normal package of tobacco is 50 grams and cost you almost 6 euro's)
Hi, I'm Dutch.

No matter where you buy tobacco in this country, all the prices are fixed: 5 * hotel, supermarket, tobacco store.. all the same price (by law).

If you want cheap, don't buy the regular well known brands, but go for something less famous. Elixyr, West and JPS are slightly cheaper. Ruba and Jacobs are cheaper but not tasty.