Credit: Splash News Online

Nick finally put a ring on Vanessa's finger -- but how much did it set him back?

On Friday, Vanessa Minnillo debuted her engagement ring from fiance Nick Lachey. A source told that the ring is a platinum, Asscher cut diamond with trapezoids on the side from designer Bader & Garrin.

Jewelry expert Michael O'Connor tells that the center stone is approximately 4 carats. "The platinum setting...makes this a classic ring, but the trapezoid diamonds give it a modern twist," O'Connor adds.

Estimated price tag: $125,000.

Minnillo, who turns 30 on Tuesday (Lachey turns 37 the same day), "loves" the ring, a source tells Us.

The newly engaged pair are "so happy, doing great," the source adds. "Thrilled about it."