What kind of legal costs am I looking at if my company has stopped paying me and I'd like to file for constructive dismissal and get owed severance etc as well as the wages owed to me? If by chance these legal fees are dependent on the severance payout, then what happens if the company goes bankrupt? I'm under the impression that the board of directors would on the hook for unpaid wages but not for severance etc, and I'm worried I'd only walk away from the case with legal fees I can't pay if the company goes insolvent.

Legal fees are very hard to predict as they are dependent on many factors which are hard to predict. For example, whether the employer wants to negotiate, the complexity of the case, the length of the litigation and many others. You are correct that directors are not on the hook for common-law notice, only for unpaid wages. As such, if your employer was bankrupt and you did not receive common-law pay instead of notice, your legal fees may be written off if you were working on a contingency agreement. If you were working on an hourly basis, you would still be responsible for paying the legal fees.