It could be your syncro on it's way out.
I bought a GT with both third and forth gears smashed to bits, fifth and sixth closley following.
I opted to go for second hand box, but a rebuild would have cost similar money.
The syncro will cost shirt buttons, but the labour involved to renew it will make it an expensive job. You may as well throw a new clutch and slave cylinder in while your at it. It all adds up.
Your best bet may be just to stick it on eBay with the fault mentioned and let it find it's own value.