The money would come from Mikhail Prokhorov (aka "New Investor"), additional financing, and new equity from Forest City Ratner or third parties.
FCR told investment analysts earlier this month it planned to invest $200 million in equity, but, as Brown writes, " it's not as if developers generally have $200 million just lying around." (Forest City hasn't yet publicly commented.)
And it's not clear to me what role the unmentioned taxable junk bonds would play in this.
From the Barclays Center Official Statement:
As one of the Vacant Possession Release Conditions, and as required under the Arena Lease Agreement, ArenaCo will be obligated to pay or cause to be paid the Additional Rent Amount (presently anticipated to be $324.8 million, which amount may ultimately be reduced to reflect prior expenditures made for the Arena Project and included in the budget for the Arena Project on and after November 1, 2009) to the Issuer (for deposit with the PILOT Bond Trustee). Such amount has been estimated by the Independent Engineer to be sufficient to pay the Completion Cost, but a portion of such amount (not needed for construction) will be used for the funding of part of the Series 2009 PILOT Bonds Strike and Liquidity Reserve Requirement. At the time of the issuance of the Series 2009 PILOT Bonds, ArenaCo will not have funds sufficient to pay the Additional Rent Amount, but ArenaCo expects to raise sufficient funds prior to the Arena Project Effective Date from one or more of the following sources: (a) the proceeds of the Purchase Price payable by the New Investor (for a description of the Investment Agreement and definitions of such terms, see "PROJECT PARTICIPANTS--The Arena Developer, ArenaCo and New Jersey Basketball--Anticipated New Investment"), (b) additional financing at one or more of the parent companies of ArenaCo, including but not limited to Arena HoldCo, (c) additional equity contributions from the Arena Developer, its parent companies, and/or the New Investor, (d) additional equity contributions from the third parties or (e) any combination of the foregoing, in each case subject to the receipt of any applicable NBA approvals. Although ArenaCo expects that the necessary funds will be timely raised, there can be no assurance that the funds will be raised or that the amount of such funds will be sufficient to make the full payment of the Completion Cost. In the event that the Issuer has not received the Additional Rent Amount from ArenaCo for deposit with the PILOT Bond Trustee at or prior to the Outside Commencement Date, or if such receipt does not occur within five (5) Business Days of Vacant Possession having been achieved, proceeds of the Series 2009 PILOT Bonds will not be applied to the construction of the Arena, and the Issuer will be required to redeem the Series 2009 PILOT Bonds at 101% of (i) with respect to the Series 2009 Current Interest PILOT Bonds, the Amortized Value thereof plus accrued and unpaid interest thereon through the Extraordinary Mandatory Redemption Date, and (ii) with respect to the Series 2009 Capital Appreciation PILOT Bonds, the Accreted Value on the Extraordinary Mandatory Redemption Date. See "THE SERIES 2009 PILOT BONDS--Redemption--Extraordinary Mandatory Redemption." If such a mandatory redemption is made, Bondholders will not obtain the expected return on their investment in the Series 2009 PILOT Bonds and may not be able to reinvest the proceeds from such a redemption in an investment that results in a comparable return.