how much is this guitar worth?

Greco f 250 might be a copy of Gretsch design of guitar....but it is uncertain as you provide no pix.

In my poking around, the only info I could find is this history of Greco guitars, which may or may not be of help to you:

Here's the link for the following info:

http://en.wikipedia.org/wiki/Greco_guitars

History
Kanda Shokai was established in 1948 and the Greco guitar brand name was started by Kanda Shokai in 1960 [1]. It was not until 1966/1967 that electric guitar production was begun in earnest with the introduction of Greco Telecaster like models. Kanda Shokai at first used the Greco brand name for the solid body models and used the Canda brand name for their Acoustic models which was based on the company name Kanda (Canda). Also in the mid/late 1960s, Kanda Shokai produced Greco branded guitars based on Hagström and EKO designs for Avnet/Goya in the USA and these guitars were made by the Teisco and FujiGen guitar factories and were very similar to the late 1960s Ibanez guitars based on Hagström and EKO designs.

In the early 1970s Kanda Shokai made Greco Gibson like models with bolt on necks which were very similar to the Ibanez Gibson like models at that time and most of these models had a Greco logo that looked more like "Gneco". By the mid/late 1970s most Greco Gibson like models were being made with set necks and open book Gibson headstock designs. Some other Greco Gibson like models from the 1970s had a different headstock design (more like a Guild headstock design) that had a Greco logo with equally sized letters.

Starting in late 1979 the Greco Super Real Series were introduced which were high standard replicas of Gibson and Fender models. In 1981/1982 the Greco Mint Collection was introduced which continued the high standard of the Super Real Series. In 1981/1982 Kanda Shokai and Yamano Gakki become part of Fender Japan and Kanda Shokai stopped producing it's own Greco Fender replica models. Since the end of the Greco open book headstock Gibson replicas in the early 1990s, Kanda Shokai have produced various models using the Greco brand name such as the Mirage Series (similar to the Ibanez Iceman), various Gibson copies (not using the open book Gibson headstock design), Violin basses (VB), Zemaitis Guitars and various other models as well.

Some notable guitar players that have used Greco guitars include Ace Frehley who used Greco Les Paul replicas when Kiss were on tour in Japan and Elliot Easton of The Cars.


[edit] Electric guitars
Greco has produced a lot of different models over the years and this is a listing of the most common models.

Gibson like models
EGF - Les Paul Flametop models.
EG - Les Paul models.
EGC - Les Paul Custom models.
PC, RR, JS - Les Paul Custom (Phil Collen, Randy Rhoads, John Sykes) models.
EJR - Les Paul Junior models.
EGS - Les Paul Special models.
SS - SG models.
JP - SG (Glenn Tipton Judas Priest ) model.
FV - Flying V models.
MSV - Flying V (Michael Schenker) model.
TB - Thunderbird Bass models.
EB - EB-3 Bass models.
MM or MG - Melody Maker models.
SA - ES-335/ES-334 models.
FA - ES-175 models.
L - L-5 models.
Fender like models
SE - Stratocaster models.
TL - Telecaster models.
JM - Jazzmaster models.
JG - Jaguar models.
JB - Jazz Bass models.
PB - Precision Bass models.
Other Models
M - Greco Mirage/Ibanez Iceman models. Also other Greco/Ibanez models such as the Korina 58 Flying V and Modern as well.
EX - Greco/Ibanez Explorer (Destroyer) models as used by Eddie Van Halen. The Greco Destroyer used U-2000 pickups and the Ibanez Destroyer used Super 70 pickups.
BM - Brian May-based models.
RG - Rickenbacker-based models.
RB - Rickenbacker-based Bass models.
VB - Violin bass models.
RJ - Gretsch-based models.
WF - Gretsch-based models.
AP - Dan Armstrong (Ampeg)-based guitar models.
GO - "Neck Through" guitar models. The Greco/Roland G-808 guitar synth is very similar to the GO1000 model.
MR and MX - Ibanez Artist-based models.
BG - Boogie Fender-based models.