If you are able to source a manufacturer and develop a good relationship, that manufacturer may be willing to provide manufacturing feedback and cost information on various aspects of your design. As I mentioned, sourcing a supplier during the design phase can also be extremely valuable for numerous reasons: 1) you can gain cost feedback on different design features, materials, packaging, etc., 2) you can also understand how various design decisions will impact tooling costs, molding costs, lead times, and minimum order quantities, 3) when you are ready to source suppliers to quote, this factory will already be intimately familiar with your product.
Some factories may be willing to provide feedback when there is no prior sourcing relationship, but this will be very hit or miss. You can try this, but you should know that you may end up turning off a good manufacturer because you approached them with a design package that looks unprofessional. With this approach, you may simply need to contact a lot of manufacturers until you find one that can work with you on these issues. Alternatively, you can find references to hook into networks of good factories that you know will respond positively.