If you will be requiring a train+bus combination (it depends on where exactly in that area you are going to), and/or making additional journeys around London, and/or also using the ticket at weekends, then a Travelcard may be cheaper.
If not, and it's just a basic 5 day a week requirement for simple journeys to Kentish Town, PAYG is £1.90 per single (same fare at any time according to TfL's single fare finder) to Kentish Town, assuming a 5-day week, that's 10 journeys per week, assuming 9 weeks then that would be a total of £171 for 9 weeks for your travel between St Pancras/King's Cross & Kentish Town, in addition to a Cambridge to London Terminals ticket for 9 weeks (1 month 30 days, assuming July/August which have 31 days in each month) priced at £768 would give your total travel for 9 weeks of £171+£768 =£939.
But a Travelcard for 9 weeks (1 month 30 days) is £978.50, so the potential saving is £39.50.
But that extra £39.50 does give you a lot of extra validity and you don't have to worry about PAYG balance etc.
(Note that the calculations above assume 9 weeks, but for a season to last 9 weeks, the price varies depending on the number of days in a month, so if you started in June you have to pay for an extra day on the season. I also assume you start work on a Monday and finish on a Friday).
