Trading can be expensive as you end up paying for the player in your own squad. and the player you give in exchange.
The cheapest way to pick up players is as 'Free agents' during the last 3 negotiation rounds in the season. You don't have to play their current club a transfer fee, just agree a salary with the player himself.
Players available for this kind of deal will be in their last season under contrack (Remaining Time = 1). You only get these players at the end of the season though, when their current contract runs out.
Basic details - you start with 2000 pts and can raise more by putting useless players from the default squad on the 'Release List' - so other clubs will make you offers for them.
I thought the rules of ML meant you can't go beyond -14000 pts at any point, and can't start a new season in the red. The actual numbers are in the ML game under the 'About the Master League option'.
The first weekly salary in ML that you pay to the default squad is 638 pts, but if you reasch Div1, because that's twice as long as the Div2 season, the weekly wages are spread out, and you can afford to pay higher salaries.
Cup runs are great for piling up pts because you pay players weekly, after the league games, the cup games are 'free' (no salary deducted after those games).