you can find cheap prices here:
https://tldes.com/
These days it's also Black Friday + Cyber Monday and most domain registrars and hosting companies offer great discounts, I'm pretty sure that whichever site you're going to buy domains from over the next 3-4 days you're going to get amazing deals... well, for the 1st year at least
And yeah, renewal costs are scary sometimes, I've fallen for this trick myself so you need to keep an eye on this

... Usually, though, the big domaining / hosting companies do have a "renewal and transfer prices" page somewhere, you just have to look it up on their site and consult it.
For example, for Namesilo (where I buy most of my domains), if you click on their pricing page (namesilo.com/pricing) and then you click on the renewal tab as shown in this image, you can see the renewal costs for your TLD:
View attachment 301275
I don't know what you mean by "keep them properly", though... Once you buy a domain (well, rent it, technically, since you never own that domain unless you keep renewing it) you can let it sit idle in your account for as long as you want. If you want to park it you can do that too, I suppose... Parking is meant to generate some revenue for your domains if you "park" them with certain companies, but in my experience parking is not worth it because the revenue is very low and you'd have to have great domain names in order to make good money with parking. I think it's much better to just use the domains, just put a website on them and promote it / SEO it and you can make more money than with parking