Is adding rows better then adding columns in case their value is not consistent?
First, it's about normalization. Today you have max 30 special products, tomorrow (or after 10 years for that matter) you might need another extra 5. You will need to write more code, but you will have greater flexibility after all. At least you won't need to code (or design) 30 columns!
You can have a schema like this:

CREATE TABLE Product (ProductId...)
CREATE TABLE Customer (CustomerId...)
CREATE TABLE CustomerProductPrice (ProductId, CustomerId, Price...)


In the third table you can put price for product/customer, but remember to create a unique constraint on foreign key columns like ProductId, CustomerId.