This app was mentioned in 6 comments, with an average of 2.50 upvotes
Here's my belief-
A PS2 game should be $15 and under, on average, except for special circumstances. There are a few things that can (IMO) affect the price; condition of the cover art and manual, limited/greatest hits/standard version, and rarity. Above all else it comes down to how much I want it, and how much I could get it for online.
When out in the field, I use this app exclusively. https://play.google.com/store/apps/details?id=com.jgrue.vgpc&hl=en It'll show you how much you should pay for a game, and how much you could get it for off of Amazon.
But realistically, it all depends on how much you'd want it. Ico is tough because it's hard to find in the wild in good condition, in most places, same for FF9 (at least to find a non-greatest hits). In general, the four games you named, none of them should be over $15, otherwise you could just amazon them.
But now, if you're trying to make friends with a retro store owner, sometimes you have to pay a bit more.
I use two apps, This when I run across something in the wild. And this to track my game collection.
From there I can download an excel format from VGTP. The big things for me are the ability to scan barcodes on games for current price trends, and to see if I currently have them in my collection. I do an inventory of sorts every 6 months, so being able to maintain my collection on a smart phone is key. Especially when scanning lot of games.
I keep the excel sheet just for sorting and reference and to have a nice hard copy at home. But honestly, when I'm out and about it's just easier to scan and have it tell me if I own it.
As for bulk purchases, I tend to go the route of overpaying slightly for items I might want, to pull them out of a bulk deal. Not everyone selling in bulk is trying to hide a gem, or even know what they're actually selling, so it's a matter of talking with them. If they won't break up, I usually walk. Unless it's a grail game, I can find it other places. (Don't get me wrong though, if I think I can make some money of duplicates, I will).
Mostly I collect by condition. If the game is immaculate, and I don't have it (or don't have that good of a copy) I buy it. I have this idea in my mind that I'm rescuing games from a horrible fate.
It's not for managing your collection or keeping track of anything, but the unofficial VGPC app is great for checking prices on the go.
One thing you could do to manage your collection is just make a Google Drive spreadsheet. Document your collection from your PC and view the spreadsheet from the cloud on your phone. You could make a sheet for each console and set up columns with filters. I like this method because you don't have to rely on some app that might go poof.
I use Video Game Price Charts by Josh Gruenberg https://play.google.com/store/apps/details?id=com.jgrue.vgpc
If you're an android user the following two apps were pretty much essential for me when browsing a store's inventory:
Also one very nice feature of the Price Charting website is the eBay snipe tool. Which will show you auctions that are about to end where the price is lower (you can set the amount) than what it's typically going for - ie good/great deals. If you allow it to post browser notifications they just show up in the lower right hand part of your monitor as long as you leave the tab open. I usually filtered it down to just NES/SNES/GC since that's what I was looking for back then.
And if you really want to crawl down the rabbit-hole, check out /r/gamecollecting It has lots of great guides about repairing/cleaning old consoles & carts, where to find the best deals, what to avoid, along with posts from other redditors who have scored finds that will make you insanely jealous.