It is quite tricky to give an exact figure because different images take differing amounts of time to complete. One of the factors in art and illustration is the cost of materials - I always use the best available to me for the simple fact that it creates the best images.
A popular commission I've had is large A2 size canvas acrylic paintings (like the hot air balloon). These take approximately 5 - 6 hours and costs around £80 - £100
For other illustrations, in terms of cost, it really depends on the complexity of the images - the two examples below are from children's books. A full-page colour A3 size image (like the woodland creatures) around £50 - £80. Something simpler - just the image, no background (for example, the monkey and frog) would be around £25 - £50. A lot of children's books have a mixture of both, there would be flexibility for bulk ordering (book illustration etc).
For something large and bespoke like the Castrol image (produced on computer) the cost will need be negotiated.
I am happy to produce work similar to seen on this site (in gallery) , or produce paintings or illustrations to fit your requirements.
If you would like any further information or if you would like to commission my services please contact me.