The better question, perhaps, is whether the ICANN envisioned in the budget is the ICANN that was endorsed in the reforms implemented in 2002. I'm not so sure it is (though the reform conversation was notable for the fact that it took place largely without consideration of the actual cost).
There are in fact some very relevant statements concerning actual costs of the reformed ICANN in the 2002 materials. I'll go into a bit of detail below to elaborate on the sequence, but in summary the Evolution and Reform Committee ultimately concluded that the funding level of the current budget (rather than the nearly doubled funding level in the proposed budget) was sufficient to support an ICANN that fulfilled an appropriately prescribed mission.
In launching the evolution and reform process in February 2002, Stuart Lynn identified "Too Little Funding" as one of three categories of "major problems" that ICANN was facing. Although the full section is worth reading, Dr. Lynn summarized his views of appropriate funding as follows:
Perhaps even more importantly, the ICANN process as presently funded will never be able to fulfill its intended coordination and consensus building tasks, its IANA and other technical tasks, its security responsibilities, its legal coordination and contract monitoring tasks, and its management tasks. Furthermore, costs are increasing even to pursue its current activities. Overall, the ICANN process is understaffed by at least 10-12 fulltime employees, and possibly more [the budgeted staffing level was then 21]– depending on what it is expected to accomplish. A fully funded ICANN probably requires an operating budget of 300-500% of its current budget level [the budgeted expense level was then $5,030,000], plus funding for significant one-time expenditures if funding of root name server operators and the establishment of appropriate reserves are included.
The Evolution and Reform Committee was created by the Board at its March 2002 meeting in Accra, with instructions to begin by crafting a proposed mission statement and making recommendations for reformed ICANN structures and procedures that would fulfill that mission. After considerable community discussion, on 20 June 2002, the Evolution and Reform Committee issued its "ICANN: A Blueprint for Reform", summarizing its recommendations to the Board. The Blueprint began by proposing statements of ICANN's mission and core values (changed only slightly in Bylaws Article I as now in effect). It then recommended a set of structures and procedures adapted to fulfill the mission. The Blueprint then made these comments about the required level of funding for these structures and procedures:
The Board should pursue a funding mechanism that requires those registries and registrars with whom ICANN has an agreement to forward to ICANN a per-name fee (estimated to be about $0.25) adequate to meet the needs of ICANN as reformed, including the building of adequate reserves. This fee is collected by registrars and/or registries as appropriate on behalf of all beneficiaries of the ICANN process.
After the adoption of the Blueprint, the implementation process proceeded in earnest. While most of the effort was devoted to crafting appropriate bylaws, the Evolution and Reform Committee continued to assess required funding levels. In its Final Implementation Report and Recommendations, the Committee commented on funding as follows:
The Blueprint suggested that the way to provide the funding required to support the activities of ICANN would be to provide that 25 cents of the price paid for each domain name registered in those domain name registries that have entered into agreements with ICANN be earmarked as direct funding for ICANN. The ERC, based on an analysis presented by the President, now believes this figure is higher than required to achieve the necessary goals. In the steady state, a maximum passthrough equivalent to 17 cents per domain name should suffice to cover operational requirements, including the additional costs implied by the proposed reform activities (estimated to require the addition of 7 FTEs to the ICANN staff). This number should, in our view, be appropriately restated every year to ensure that the total revenue to ICANN does not increase, except for inflationary reasons, as the number of applicable domain names grows (unless otherwise justified and approved as part of the annual budgetary cycle). By comparison, the current ICANN budget provides for a passthrough of approximately 13 cents per domain name. In addition, the ERC recommends that the passthrough be increased an additional 3 cents per domain name (to a total of 20 cents) until such time as ICANN's audited unrestricted reserves rise to the level of one year's operating costs, following which this additional amount should be discontinued. We note that other funding sources, such as voluntary contributions from ccTLDs not under agreement, may also contribute to the rise in these reserves.
Section here.
These recommendations were essentially implemented in the next budget (the current one for 2003-2004), which provides for $8,273,000 in expenses, with registrar fees then projected at 18.9 cents per name.
The above history seems to bear out your initial assessment ("I'm not so sure it is") that the budget now proposed is not consistent with the mission defined by the evolution and reform process.
A concerned reader

