What is the best time to patent our product?

Assuming that you have a new and novel invention, and further to Steven Weinrieb's comments, you will want to file an application for your inventive elements before any public disclosure or offer for sale. Not only does this strategy provide you the ability to discuss your invention(s) with investors, outsourcers, future employees, etc. with more peace of mind, but if you do not have a patent application filed (e.g. provisional, non-provisional, PCT, foreign application, etc.) before public disclosure/offer-for-sale, you will preclude your ability to get a patent in certain foreign countries. There are very few countries that offer a 12-month grace period between public disclosure/offer-for-sale and application filing (I believe that the US and Canada are the only two that have the grace period). Filing in other countries may not be on your mind right now, but it could become very important for certain inventions.

With respect to budgetary considerations of "when to file", I usually advise my budget-conscious start-up customers to find initial funding (e.g. family/friends) to file an initial patent application (often a provisional done properly but would be better to have a non-provisional filed - a separate discussion) on the foundational aspects of the product, then put budget into other aspects of the business, talk to investors, etc. and then revisit a solid patent strategy once things get rolling.