Below are past card alter projects I've done. I have only done Magic: the Gathering cards so far, but I'm definitely up for doing alters from other games!
Basic pricing for card alters:
$25-30 for extended art (where some features go outside the original art frame)
$30-50 for fully borderless (borders are painted over but card text, name/title bar are still visible)
$50-70 for highly detailed alters (full art, complex borderless)
These prices are an example to provide a baseline of what my pricing is like. Your actual commission price may be more or less than the above depending on what you're looking for.
Here's a helpful blog post with guidelines on the tournament-legality of alters in Magic: the Gathering :