Domain Monitoring

Some questions arise:

1. How many domains exactly is the large portfolio? "large" is very unclear
   
2. How often do you need changes to be reflected (hourly, daily, monthly...)

These factors influence the price a lot.

Around 12,000 domains with daily updates.

Simply from the volume of whois queries needed, I guess it would cost about $400 per month.

Here's as bulk whois provider, probably the most well known:

https://whois.whoisxmlapi.com/pricing

I don't have a suggestion for a monitor service, but things are going to be roughly in that area.

Edit: Depending on complexity of monitoring it might be a bit on top of the data cost

Side note I used to have in the past a platform that was doing something quite similar on a large scale.

I'd be able to help under a lower cost, shoot me a DM if you want.

Lot of possibilities there, but afaik nothing free for that amount of domains.

DomainTools and DomainIQ have such kind of monitoring. I am using desktop software Domain Punch for that, although I am managing my own portfolio with their hosted product Watch My Domains SED. Also, they have some freeware software, so check that.