You don't have to spend a lot to enjoy the benefits of hammock camping. It sounds like you already know the jargon and have probably seen some great hammock kits out there (e.g., Warbonnet, Hennessy, etc.). These kits are great because, generally speaking, they have all the major components (rain protection, bug protection, hammock body) combined into a system. They can also be expensive.
I went with a Hennessy for my first camping hammock. It was a good transition for me because it had all the major components and it helped me transition from a tent. I had never really tried tarp camping before, and that was a new experience, but the integrated bug netting helped make the whole system feel more "normal". Over time, I found, or became accustomed to the simplicity of hammock camping and began using basic, less expensive hammocks. When family or friends asked, I would recommend they start cheap and see how they like it first. There's no question about a hammock's comfort, but as a replacement for a tent, I realized that not everyone would transition easily.
You can find great hammock systems for a lot less if you shop around. Grand Trunk offers a hammock with zippered bug net for around $80 (less if you look on discount sites). Add a Guide Gear 10x10 tarp for $30 and you've got a great, year-round shelter.
You can use your old sleeping bag and pad in a hammock just like in a tent, so there is not cost differential there. A lot of people start off with using pads in a hammock. A blue, closed-cell foam pad from Walmart costs $8 and can get you down to 30°F (-1.1°C). Double up on pads when it gets that cold and you're still well below the cost of a down under quilt.
I started off with pads and still use them from time to time, but once I purchased an under quilt, it's hard to go back. Under quilts are really worth the price and will last years when taken care of. Invest in good insulation and you won't regret it. For hammocks, I'm convinced that nearly all the manufacturers are nearly equal in quality, so shop around for the best price for the size of hammock you want.
If you're looking for something specific, e.g., a certain load capacity or ultralight materials, that will narrow down the selection.
I have a lot more information, including a complete manufacturer list, in my new book on hammock camping: The Ultimate Hang.
Author and illustrator of "The Ultimate Hang: An Illustrated Guide To Hammock Camping"