Any injection mold manufacturer can quote a low price mold, but it takes a committed partner who is familiar with your company and processes to engineer the lowest cost solution. That means an up-front investment in time and expertise which only comes out of strong working relationship.
The best practice is to avoid preparing the Excel sheet containing 3 to 4 different mold makers quote and make a rating based on the price. Visit the mold maker plant and check the facility available with him, his existing customers, past customers. Check the molds which are under manufacturing in the shop, see the procedures followed, look at the sample components made from the molds supplied by the mold maker. All this will give you an idea on the capability and the quality levels of the mold maker.
The two biggest determining factors of molding sink marks are product design and processing with the assumption that the tool is functioning correctly. With that being said there is a fine line between the two. You can over compensate for poor product design with molding parameters, and vise versa. If you over pack a part you can begin to effect it dimensionally or ride on the fringe of flashing your parting line. You can very easily make a part that is too big by trying to pack out molding sink marks that are caused by poor product design. The other down fall of over packing is a small processing window.