There is a world of difference between a 12 bore Holland paradox and a 10 bore Holland paradox. An 8 dram 10 bore gun is a no joke elephant gun. Most Holland 12 paradoxes are top lever and you would probably pay a premium for an underlever 12 Holland if you could even find one. There were many other makers of ball and shotguns. The Fosebury style rifled choke guns are at the top in my opinion. You might find a rifled choke non Holland 12 bore gun for under 10k. I would doubt you could find a serious 10 bore gun for under 10K. If you have an interest you will want to buy Roger Lake's book on the Holland paradox.
www.Paradox-Horsley.com