What are the standard prices for pet game art?

I'm sure you expected this, but the answer depends on a lot of factors! I've been making petsite art for a decade+ and I'll list some of the things below that influence my prices:

The level of detail in the rendering
Something that only needs 1 layer of shading is far easier to produce than multiple, highly detailed layers of shading and highlights.
The style of the characters
Cartoony is easier than realistic!
The dimensions of the artwork
Making a tiny 400px graphic is way easier and faster than being asked to make a 300dpi print-ready file.
The amount of design work required
Drawing a wolf doesn't take as much design planning as a completely original critter does.
The total number of graphics
This is mostly relevant for sites that have growth-stages, but an artist may be willing to offer their services at a discount if you intend to commission a LOT of art or give them a long-term job.

It's worth it to consider the above points and decide what kind of art style you want to go with from the outset. This will help you locate an artist that adheres to your preferred style, or will help your artist come up with an appropriate price quote if you can provide details like this for the style you're looking for.
On the flipside, I have also had people tell me their budget first, and I showed them the type of quality I'd be able to give them within that budget.

All that said, VPS artwork largely tends to be an "entry-level" field. This means you're likely to find many artists who undervalue themselves, or fledgling artists who produce amateur work. Most VPS artists I know make and sell their art for far below minimum wage. Those with more professional experience are usually trying to get hired by apps and game companies with deeper pockets.

When I first started selling my VPS art (we're talking a decade ago), I was fine selling completely finished pieces for $15-$20. I was just thrilled that anyone would pay money for my art at all. As my art improved, I started asking for more compensation, and $40-$60 for a finished semi-realistic original design wasn't unreasonable.
These days I charge $80-$200+ for a single, non-commercial image. For commercial work like VPS designs, I've left the flat pricing behind and now do everything for an hourly wage of $24-$30/hr, with print-ready graphics usually taking me a minimum of 5 hours. Though that's just my own personal pricing system. Few artists get those kinds of wages for VPS art, because again, it's generally a field for beginners and hobbyists.

In summary: You can pay whatever you like, but you get what you pay for! If you want low prices, you're likely to get amateur art that may need to be "revamped" in the future, costing you more in the long run. This isn't necessarily a bad thing since revamps are a favorite topic of VPS history for me, but it's something to keep in mind! If you think an artist's price quote may be a rip-off, feel free to ask them what variables are contributing to the price. I'm always happy to explain such things to my non-artist clients, and they may be able to work with you to find something you can afford.