For comparisons to analogous heavy rail projects, phase 1 of LA Metro's Westside Subway extension is $3 billion for 3.9 miles. This is one of the densest stretches of LA, and there was a lot of NIMBY opposition from Beverly Hill residents. For NYC 2nd Ave Subway, they spent $4.5 billion for a 1.8 mile extension. This is the densest stretch of NYC and there were similar complaints from Upper East Side residents. But of course the MTA is terribly mismanaged and completely blew up costs of the project, such as stations each costing $400 million compared to LA's $120 million stations, and tunnels being 200 ft below ground compared to 100 ft.
A Brown Line extension project will likely be more similar to LA's Westside Subway for several reasons. One is that the Brown Line can't dig too deep without disrupting the storm drains, so 100 ft is the likely tunnel depth. Beverly Hill and UES NIMBYs had campaigns and plenty of money to sue the extensions and delay the project, resistance from any Albany Park residents will be no where near as extreme. LA also has to build new turnbacks and new yards, while the Brown Line would be borrowing existing yards from the Blue Line. $1.8 billion for 2 miles shouldn't be entirely unreasonable.
Sources:
https://nymag.com/intelligencer/2019...ure-costs.html