Generally speaking I agree with Jayde, but I myself don't really have any hard and fast rules about game buying.
I'm more inclined to buy audio games anyway because I like to support the devs, but the more expensive the game, the more factors must be taken into account before I buy it.
things I considder include: a game's plot and complexity, if there are exploration elements, whether extra content (user created levels etc), is available, how varied the modes and choices in gameplay are, if a game has ongoing in game plot or not, how many possible goals etc there are in the game besides just accumulating score, how long it will take me to complete the game, how rich a game's audio is, the game's atmosphere, how generic the sound effects are, whether a game uses human or synth speech, how the game stacks up against similar free or commercial games, whether the game is available via download, and if the game's demo is replayable (I am qquite willing to reconsidder my decisions on games, ---- I actually tried Trek 2000 three times before I eventually decided to buy the game).
The lower the game's price, the more allowences I will make if it fails in certain areas.
for example, I decided against buying ten pin alley because, ---- despite the very nice audio, I found the game play slightly repetitive, simply centering an audio signal as in Crazy darts from Bsc, with many extra stuff about scoring etc.
It struck me that none of the extra game options made huge amounts of difference (bowling left to right instead of right to left for instance), and anyway, the ultimate end of the game is just a continual match winning scenario which personally doesn't appeal to me, and unfortunately sinse it's usually me that visits my friends rather than the other way round, I couldn't make extensive use of the various multi player options.
If the game had worked out at about 7 pounds, fifteen dollars or so, I would probably have bought it.
Similarly, because the level layout doesn't change, and the gameplay is the same from stage to stage (no extra weapons etc), I haven't yet bought packman talks, though it's possible if the game's price drops in the future that I will.
A game would have to be very good indeed for me to considder paying over about 40 usd, ----- or about 25 pounds in my case, and I physically don't know if I'd be willing to go up to about 50 usd for a game.
As a rough price guide to audio games, you might find this list from Pcs games to be helpful.
With our dreaming and singing, Ceaseless and sorrowless we! The glory about us clinging Of the glorious futures we see,
Our souls with high music ringing; O men! It must ever be
That we dwell in our dreaming and singing, A little apart from ye. (Arthur O'Shaughnessy 1873.)