I'd go for the third one in the series:

https://www.gog.com/game/populous_the_beginning

I know I hated the first one simply because of that tiny window.

You know you can search by genre, then filter by price, right? That way you can see for yourself what's on sale cheap. Would likely be a better option to do that, then ask about any games you see that may look interesting. Just asking about cheap games or cheap RPGs is likely only going to end up with a lot of suggestions of things you've already played.

Ha! I did play the demo version only, quite long time ago.. As far as I can remember the demo of Populous The Beginning was bundled with SimCity 3000 CD, the game my second half used to spent hours and hours almost every bloody day ; )
I have to play the full version yet, already waiting in my library.
As for tiny windows.. I'm use to play on screens from 9" to 43" actually, on the biggest one every game looks great for me : D

edit:
I think everyone interested in the game should grab whole serie at once. For this price it's a steal..
And there are many more cheap as chips games one the sale. Hard to imagine for someone who had to work a whole week to buy Quake ; )

But tell me, please - is the Populous still playable today?
How hard is it for someone not very good at strategies?

Totally playable. I just played it recently.
Beat AI in Populous is never a problem for me.
I am not very good at StarCraft 1.

I did play the old one only for a while, doesn't seem overcomplicated if you ask me.
Absolutely worth giving a try I'd say : )

Not my thing, I was thinking more along the lines of Torchlight, Baldur's Gate SoD, Foxtail - games that are cutish or are doing nice on modern systems.
I also own Trine 1&2, so any recommendation along those lines are also welcome....

Got STALKER games on a sale recently. Do you think the mods bundled in the "Starter Pack" good enough for SoC? I am aiming for a close to vanilla gameplay, albeit with bugfixes, visual improvements and minor quality of life tweaks.

BeatriceElysia, have you played Bastion? It is a great game, and is currently onsale for $3.79.

No, but I'm planning to, since I got it on last sale (wishlist sale).

Excellent.
I would also suggest getting Double Fine's games : Psychonauts, Stacking and Costume Quest, if you don't have them already. Each is currently below $2 mark.

Played last year. Unforgettable experience. It's not only one of the best cRPGs ever created. I'd say it's one of the most interesting products of culture I know. That setting and immersion... you'll dive deeply into another world and feel it with all senses.

S.T.A.L.K.E.R... I can't remember how many years I was waiting for that one.. Grabbed at release excellent edition with map and some other goodies, went back home and spent whole night in The Zone : )

Like bhrigu mentioned, Costume Quest is absolutely fantastic. Not very long, but definitely unforgettable.
If not into Bloodlines, perhaps Vampire: The Masquerade - Redemption will suit you more.

How about BrothBrothers: A Tale of Two Sons? Very short and very memorable at the same time..

Go for Fallout 1&2, I think they are on sale right now.