I can totally understand why you might think tunnel should cost 5. I remember back when I was discovering the game, and hinterlands was the 2nd set I ever got, I had a phase where I thought tunnel was AMAZING. It's so satisfying triggering the gold gaining effect kind of like activating a menagerie or forging away all the junk in your deck. You have this vision of activating a couple tunnels over and over again and just loading your deck up with gold after gold after gold. Hinterlands has some great enablers with cartographer, oasis, oracle etc. and it also gives you the last laugh if your opponent plays a militia or minion at the wrong time.
When I first looked online to see what other people thought were the best cards and first stumbled upon the annual rankings here, I was pretty surprised to see tunnel near the bottom. As you learn more and more about the game and how dramatically your strategy can change depending on what's present or not present in a kingdom, it will make sense. The sad truth, and I am a fan of tunnel, is that it's not the powerhouse you think it is. In fact, it's pretty rare that you want a tunnel.
First of all, you need an enabler. As I mentioned before, there's a fair amount of the out there (cartographer, minion, artificer, cellar, etc.). If there isn't one out there, you've got a dead victory card. If there is one, you'll probably want to spend an early buy on a potentially dead victory card when you could be buying other valuable engine pieces. If you do spend an early buy on it, you need to get it to collide with your enabler. If there isn't any great trashing available, you may feel compelled to buy multiple tunnels which, while it can certainly get you more gold, can clog up your deck and make it less reliable.
Another thing that kind of goes against what you might think- you don't always want to flood your deck with gold. A gold here and there is fine as a way of buying expensive action cards which will probably be the ultimate focus of your deck. In an engine, gold is a stop card, and so is the tunnel(s) that you've used to get it, which gets in the way of drawing your deck and leaves you prone to dud turns. In pure big money games with no +buy, sure it's great to have a bunch of gold. Of course the problem is that those kind of games aren't too common anymore with how the game has evolved. There are boards where the absolute best thing you can do in a turn is just buy one province, but usually you can be doing something even better while maintaining better end game control.
So yeah while tunnel seems like it can be uber powerful, it's usually a trap. Which is a shame because during the rare even that it IS an integral part of your plan, it's soooooo satisfying. I do think it's underrated because I think gold is a little underrated and underappreciated these days, but also 2VP for $3 is not bad at all. I'm pretty sure it's the only way you can get 2vp for less than $4. I don't see why it shouldn't cost $4 to be honest. That seems to be the sweet spot for alternate vp and nobody's opening double tunnel.