If it takes a full month to gross £3000, then value is £6000 tops. If, however, it took say 3 weeks to earn the £3000, then yes, maybe £10,000-£12,000 is appropriate. If one guy can earn the £3000 in 2 weeks the asking price would be even higher.
...You see the point? The AMOUNT of work is almost irrelevant. QUALITY determines the price.