IT was a thought which probably passed through the minds of many spectators at the Cathay Pacific Hongkong Bank Invitation Sevens last weekend: ''I bet the Hongkong Rugby Football Union (HKRFU) really makes money out of this. I wonder how much?'' The answer is an estimated $22 million. But this doesn't mean that the HKRFU is wallowing in money because of the Sevens. The union's flagship event generated income from ticket sales, corporate sponsorship, catering services and souvenirs. . But the cost of organising the tournament and the money required to support the territory's union network, development and tour and training programmes will absorb most of the funds earned from the Sevens. HKRFU Finance Director Mr David Bruce said this week that while the Sevens regularly makes a substantial profit, Hongkong is still the poor relation, in financial terms, compared to the major unions of Britain, New Zealand and Australia. ''Last year, the rugby union was a profitable concern,'' Mr Bruce said. ''However, costs are increasing all the time and to remain in profit there will have to be some tighter financial control. ''What has to be remembered is that unlike other countries, we have a development programme which is very expensive to maintain with 10 full-time staff. ''Also the national team takes up a lot of money and the union financially assists in the hiring of pitches for matches. ''This is very expensive and does not happen in other unions.'' It has been the union's policy not to reveal their accounts to the public but they are believed to have made a $5 million profit for the 1991-1992 season. This was practically exhausted by the time last week's Sevens started. Its main source of income comes from sponsorship, the Sevens and the Sports Development Board, although rugby is not one of the Board's target sports and the annual subvention does not exceed $500,000. The union has more than 20 sponsors who commit themselves in varying degrees to union activities, including mini-rugby, the national league, cup competitions and tours. This year the Sevens generated $7 million from the 100 corporate boxes which occupied the middle tier of the Hongkong Stadium. The cost of boxes ranged between $65,000 and $75,000. Ticket sales accounted for $10 million while further income came from sales of official union apparel and catering services. The union's chief executive officer Mr David Roberts said the cost of staging the Sevens was immense. This year the stadium fee alone was $2.5 million. ''This year's has been one of the most expensive Sevens to organise,'' said Mr Roberts. ''With the partially-built stadium, extra costs and unforeseen expenses have cropped up. It cost millions of dollars. ''We also have to pay preferred hotel rates for the players. We pay for their food and water and also their transport to training pitches the week before the tournament.'' The financial status of the union has been of public interest ever since the Sevens became recognised as one of the world's leading rugby tournaments. But union chairman Mr Stuart Leckie defended the decision to keep details of the accounts from the public. ''We regard ourselves as a club and organisation serving the rugby clubs and the players,'' said Mr Leckie. ''It is not as if we are a public company. ''There is a lot of income and a lot of expenses, and I can say that any surplus that we make goes back into the development of the game in Hongkong.'' Mr Roberts said: ''The Sevens supports the development programme and ensures the future of Hongkong rugby. ''If there were no Sevens, the development programme would still exist but not to the same extent as it is now.'' This programme aims to popularise rugby in Chinese schools through training, equipment and coaching expertise. The union also has to finance the many tours undertaken by national and other teams. In the past 12 months, the Hongkong Sevens team has travelled to Sicily for the World Cup qualifiers, Dubai, Darwin, Singapore and Canberra. Last year, the under-24 squad toured Japan while the national 15s team had a three-match tour of the United States and spent a week in Seoul, competing in the Asian Championship.