The net salary is an after tax function, and that will obviously vary from individual to individual depending on their own tax coding. This is further complicated by the fact that some partner airlines operate "loan repayment programmes" whereby the salary is separated into two constituent parts. One of those parts (if the cadet elects to utilize it) results in the effective loan repayment part being treated significantly more favourably for tax purposes.
Initial placements are often on the basis that the cadet receives no salary for the placement period, but does receive a repayment of part of their bonded training costs for that period. This is typically around £1200 a month and added to that is the per diems the airline pays its regular pilots. A ballpark figure is likely to be around £2000-£2500 per month (net). If the cadet pilot is offered an employment contract at the conclusion of the initial placement period that will depend on the rates applicable with the airline concerned. At our company they would be placed on a gross rate of around £4500 per month plus typically per diems of around £1000 giving a gross rate of around £5500 per month (£66,000 per annum). That would typically provide a net rate (which might vary significantly from individual to individual depending on some of the factors already mentioned) of around £4275 per month net (when utilizing an HMRC agreed bond repayment programme.)
This is a snapshot of those figures I can currently view and it goes without saying that each and every airline will differ in respect of how they utilize these transition training programmes.