Kannan,
It would seem that you really are quite uncertain as to the options that are before you. I doubt I will be able to identify the way forward for you, but perhaps my own observations and comments may help, or at the very least stir others into contributing.
It would seem that in these challenging times, if you can see opportunities in the auditing field, then it would be wise to pursue these, and to that end you should look at one of the auditing qualifications that exist - I would suggest a nationally recognised one is the best way to go, although they are more expensive. If you want to be auditing health and safety, then you would also need, in my opinion, a recognised qualification in health and safety to allow a degree of credibility to be established. The level of qualification you choose to pursue can be determined by the particular field you would want to specialise in, arguably your degree gives you that for general auditing in the hotel and catering industry, but the safety content of your degree is likely to need supplement with something like the nebosh certificate.
You do need to be aware that the nebosh certificate, whilst being a superb qualification, is not at the standard utilised by many organisations for safety practitioner roles (some places will use it, but not many). So if your aim is to become more of a health and safety practitioner, rather than an auditor then you need to consider the diploma level qualification.
There may be merit in talking to the lead health and safety professional within your own organisation and getting their views on the best way forward within your own industry. I would almost put money on the need to increase the level of training you have in food safety if you are to focus on the catering and hospitality sector.
You ask about costs, I am not based within your area, so I am not able to give you best local guidance, but in general local colleges will give you a good price for a nebosh certification, auditor qualifications I have little experience of, so cannot comment on what is best, but perhaps another contributor will be along shortly to advise you on this. Diploma level qualifications can be expensive, but again local universities and colleges are an option rather than NEBOSH Diploma providers. Check out the qualifications links within this website to see the GradOSH approved qualifications from around the world.
Go to your local branch meetings there will be many there who will be able to offer advice, perhaps from your own sector
Anyway, I hope this has helped provide a degree of focus and help
best of luck
Stuart