I understand yesterday's win was worth £7k
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Rangers will receive £4,500 for winning yesterday's tie. They have also guaranteed themselves at least another £2,250 even if they lose in the next round. So, yes, just short of £7k in total.
The prize money starts to jump up significantly:
First round qualifying winners (116) £4,500 Losers receive £1,500
Second Round Qualifying winners (80) £6,750 Losers receive £2,250
Third Round Qualifying winners (40) £11,250 Losers receive £3,750
Fourth Round Qualifying winners (32) £18,750 Losers receive £6,250
First Round Proper winners (40) £36,000
Second Round Proper winners (20) £54,000
Third Round Proper winners (32) £135,000
Fourth Round Proper winners (16) £180,000
Fifth Round Proper winners (8) £360,000
Quarter-Final winners (4) £720,000
Semi-Final winners (2) £1,800,000
Semi-Final losers (2) £900,000
Final runners-up (1) £1,800,000
Final winners (1) £3,600,000
A cup run like that in 1974/75 would net over a quarter of a million. Whilst that may not be realistic, clubs from Rangers current level do regularly make it through to the first round proper and beyond. Reaching the first round and drawing someone like Sunderland away would be a huge boost to the Club financially.