It is very difficult to provide an estimate of the legal costs of a merger/amalgamation of Canadian charities and non-profits without thoroughly examining the issues and reviewing documents, etc.   The costs can be as little as a few thousand dollars and in some larger mergers to over a hundred thousand dollars.   Some mergers are relatively straightforward but most are not. There are many different parties or groups that can affect the amount of time involved in a merger including the parties to the merger, provincial regulators, federal regulators, stakeholder concerns etc. There are also many options in terms of how a merger can be carried out and each has pros and cons.   
In most cases that we see, the non-legal costs of the merger (such as staff time, other advisors etc) are significantly greater than the legal costs.   Having legal counsel who is very knowledgeable about non-profit and charity issues can save the organization costs, streamline/speed up the process and reduce risks and anxiety.      
Some of the many factors or issues that can determine the cost and time spent include:
-what structure is contemplated (amalgamation, consolidation, asset transfer, membership change, etc)?
-how much discussion takes place before decisions are made?
-are the merger partners all up to date in their filings and have clear governance structures?
-what is our role in due diligence? Are we going to be involved or will our client do the due diligence?
-are there significant liabilities or contingent liabilities?
-are there employees, especially long-term employees, or unions that need to be planned for?
-how quickly and completely is requested information provided by the client to us?
-what is the time horizon for completing the merger? (generally, it is easier if there is more time)
-how much internal capacity does the client have to conduct the merger vs. assistance we need to provide
-are there concerns from funders and other stakeholders that need to be addressed?
-how helpful is the other party or parties to the merger?
-in some cases, we are the main legal advisor and our client is the dominant party and we are doing most of the legal work whereas in other cases another law firm representing the other party is doing most of the work and we are just reviewing documents, making comments and suggestions
-the involvement of other professional advisors (such as real estate lawyers, labour lawyers, accountants, etc) who may do a significant part of the work to identify issues in the due diligence process and to rectify those issues. 
-what work will need to be done to justify the objects to CRA if the entity is a registered charity?
-if in Ontario will there be PGT concerns with Ontario corporations that are involved that are “charities”?
-are the memberships broad or narrow that need to approve the transaction?
-are there restricted funds/gifts held by one or another party?
-we often provide ideas to clients about minimizing legal costs and the extent to which clients utilize or adopt those ideas
-are there changes by the client in the form of the merger or their instructions?
-does the merger go through or is it abandoned before completion?
-do you require assistance post-merger with the integration issues?
We have written a number of pieces on the non-profit merger process that you might find helpful including:
Mergers and Amalgamations within the Canadian Non-Profit and Charity Sector (The Philanthropist 2009)
20 Questions Directors of Not-for-Profit Organizations Should Ask about Mergers (CPA Canada 2016)
We have prepared an online course dealing with non-profit mergers and you may find it helpful to understand the area.  You can find it at:
With respect to mergers or potential mergers we suggest having a consultation.   Generally, the cost of an initial meeting will be $500 to $1000 and at the meeting you can decide what further assistance is needed and we can discuss the costs associated with that. Often times we assist other lawyers with the specific non-profit aspects of a merger such as providing counsel on dealing with CRA or non-profit corporate issues or strategy.
A poorly planned or implemented process can be extremely costly to an organization.  It is ideal that non-profits and charities obtain legal counsel early on in the process to avoid significant risks, mistakes, wasted time and distraction.