CPACS: Research, Results, Market Comparisons, and Valuations, page-238

Good question. In addition to ttppxx's response here are my thoughts.

The 2020 sales (not peak sales) of Imbruvica was $8 Bn per year. So at 50% of that which $4Bn for Abbvie, that is a payback of their $21Bn in just 5 years (rest of the world not included).

However per below source, the sales already dropped to $7Bn in 2021.
Source: https://www.globaldata.com/data-insights/healthcare/the-global-drug-sales-of-imbruvica-1127422/

Per this next source from Oct 2023 'Sales of AbbVie and Johnson & Johnson’s leukemia blockbuster Imbruvica are taking a hit from the introduction of BeiGene’s transformative oral treatment Brukinsa. But what about the impact of the Inflation Reduction Act? In August, Imbruvica was one of the 10 drugs identified by the Centers for Medicare & Medicaid Services (CMS) that will be subject to price negotiations in 2026.' Suggest reading this interesting news post - https://www.fiercepharma.com/pharma/abbvie-estimates-value-imbruvica-will-decline-21b-because-cms-price-negotiations

From the above news article Abbvie reported Imbruvica sales for the 3 quarters of 2024 of $2.7Bn which extrapolates to about $3.6Bn in revenue for the full year. This is a 55% drop (significant) from $8Bn in 2020 even though the competitor's drug isn't pulling more than $211 Mn in revenues! So surely this isn't boding well for the Abbvie's M&A strategy.

Haven't dug deeper but still an interesting case study.

Any errors are mine. No advice.