Stats:

Difficulty:

• https://www.bitrawr.com/difficulty-estimator
• https://www.coinwarz.com/mining/bitcoin/difficulty-chart
• https://bitcoinexplorer.org/difficulty-history
• https://insights.braiins.com/en/cost-to-mine/

Here's an article that has some general info about bitcoin mining difficulty.

Network Difficulty, Share Diffiaculty and Hash Functions
https://hashrateindex.com/blog/crypto-mining-network-difficulty-share-difficulty-and-hash-functions

Hashrate:

• https://www.coinwarz.com/mining/bitcoin/hashrate-chart
• https://bitinfocharts.com/comparison/hashrate-btc-ema14.html#3y

ASIC Prices:

• https://data.hashrateindex.com/rig-index-data

A great tool to monitor the progress over the 2,016 block epoch is fork.lol's Difficulty Retarget chart:

• https://fork.lol/pow/retarget

And some great Dashboards, overall:

• https://timechainstats.com
• http://vaccaro.network:3838/bitcoin/#shiny-tab-bitcoin_pools_dashboard
• https://mempool.space/mining
• https://mempool.space/graphs/mining/pools-dominance
• https://insights.braiins.com