The furnishing of a performance guarantee in favor of the Borough of Farmingdale in an amount not to exceed 120% of the cost of installation, which cost shall be determined by the Borough Engineer, setting forth all requirements for improvements as fixed by the Board and their estimated cost. The estimated cost of the installation of improvements determined by the Borough Engineer shall be based on documented construction costs for public improvements prevailing in the general area of the Borough including streets, grading, pavement, gutters, curbs, sidewalks, street lighting, shade trees, soil erosion control devices, surveyor's monuments, as shown on the final subdivision plat and required by the Map Filing Law (N.J.S.A. 46:23-9.9 et seq.), culverts, storm sewers, drainage structures, public improvements of open space and, in the case of site plans only, other on-site improvements and landscaping provided that 10% of the total performance guarantee shall be in cash and the balance shall be in the form of a bond or other securities or guarantees approved by the Borough Attorney. The foregoing shall not, however, preclude the developer from posting a greater percentage in cash if the developer so desires plans only, other on-site improvements and landscaping provided that 10% of the total performance guarantee shall be in cash and the balance shall be in the form of a bond or other securities or guarantees approved by the Borough Attorney. The foregoing shall not, however, preclude the developer from posting a greater percentage in cash if the developer so desires.