How much would a numbered and signed print be worth by Leila Platt copyright of 1987?

You can search on auction sites (like ebay) for similar prints by that artist. Search for "Leila Desiree Platt".

Right now, there's prints with similar description offered with "buy it now" prices around $20. Price depends on your particular print, some may be worth more, but looks like average prices are that low.

(I just paid $10 for an Unsigned print of hers, and seems that was a fair price for what I got. I think it's worth it, not as an investment, but just as a decoration.)

JMS