I'll go out on a limb here and guess that one of the reasons you're reading this post is because you want to spend less rather than more...
Am I right?
While the obvious answer "Just spend less" to the question of "How do I spend less on boat stuff?" is correct, for some reason or other most folks have a problem with actually absorbing the particular cosmic truth involved or adopting it as their mantra.
So, let's work up to it in a few more easily understood stages...
Need/Want... Always ask yourself "Do I really need this?" and as long as you're honest you'll be halfway to being seriously frugal with your boat and cruising related expenses.
Once you've established that you actually need (rather than just want) something, you have to figure out just why you need it and what purpose it actually fulfills because in a world full of bells, whistles, and mission creep it's hard to see where the need and want overlap... For instance, do you really need the race start features in your instruments or do you just need to know how fast/far you're traveling? If there are features you won't need or be using maybe a less featured unit would be a better, less expensive choice.
Be cynical... Face it, people who are trying to sell you something are not always honest, seldom have your best interests at heart, and often quick to take advantage if they think they can get away with it. Since I don't expect the marine trades to find a collective conscious anytime soon, the way to sort out this sorry state-of-affairs is to learn enough about the stuff you need on a boat to know the difference between BS and reality. Knowledge being Kryptonite to almost all predatory marine pricing and services.
Be cynical some more... Sadly, a lot of the information available is not based on research and personal experience but simply regurgitated information provided by the folks who want to sell you stuff (the phrase bought and sold does pop to mind). The bottom line is you NEED to do your own research and come to your own conclusions.
Get handy... If you can repair or recycle old stuff you're way ahead of the game. Plus, the knowledge it provides about how stuff works makes you that much more resistant to falling for the new hip thang the salesman or press is telling you that you really, really need (remember knowledge = Kryptonite).
But yeah, when all is said and done, it's still just about spending less...