Without the need to contact millions of people to make the case for your election, campaign spending in support of or against an individual will most likely fall dramatically. Individual proxies may hire consultants to help raise their profile and gather more voters, but that will not be prohibitively costly nor prevent others from building support in other ways.
Negative advertising may develop to target proxies in relation to specific policies they are working on, but so long as they maintain a core group of support, they will not risk losing their position and will likely be able to rebuild support once the issue passes.
This does not mean money would disappear from politics. Instead it will be channeled through parties and issue groups. However, spending is currently used primarily is to either support, oppose or threaten a politician over a policy, meaning that spending goes for or against a candidate. Under liquid democracy, this priority disappears and the voters become the ultimate deciders on all legislation, meaning that groups will spend money in support or against policies.
Moreover, liquid democracy provides a straightforward path to developing remedies to these problem. While courts may limit overall reforms, liquid democracy makes it easy to quickly consider new ideas from a variety of sources. Self-interested proxies may still try to limit reforms, but voters would always be able to overrule them.