I did some search online, there are some articles to suggest to treat domain names as intangible assets, like trademark. Thus the depreciation will take 15 years.
If it comes to getting audited, there would be a great debate about the actual value of a domain name. Therefore, I just wrote them off as a business expense (one time) and am not worrying about it any more.
Domains might be similar in some ways to copyrights, trademarks, and other intellectual property, too. I'd ask your accountant. For a modest transaction like the one you describe, you can probably choose your preferred approach without a ton of risk. Due to the inefficient market for domain names, you could probably justify writing them down very quickly.