There are a lot of ways to look at that. 
1) When I lived in Texas I worked with an AG student that was designing 500 hen chicken coops for Vital Farms to use for free range flocks.  Vital Farms eggs sell for about $7.00/dozen wholesale at gourmet food stores.  The AG student I was working with said that each hen in these free-range flocks was worth about $75 in profit over its life span. 
So...If you are getting 256 eggs a year from your Opal Legbar and selling eggs for $7.00/dozen then they may be worth $75 (depending on your overhead).
2) If you are selling pullets/hens for back yard layers then the value will change.  Anyone can get a day old sexed pullet at the feed store for around $3 each.  People that want backyard layers don't want to pay much more than that for a pullet if there only goal is to have fresh eggs.  There is a smaller market inside the backyard market that will pay a premium for blue eggs.  The easter eggers, however, can still be found for around $3 each.  So you really have to find people who collect chickens (i.e. 10 hens each of a different pure breed) to get the premium for back yard layers.  Rare breeds, unique color patterns, heritage chickens, or other special features to the look or utilities of a breed would bring a higher value.  These rare/unique breeds may go for $10 for day-old chicks straight run and more for sexed pullets.
3) Some people don't want day old chicks.  They don't want to fuss with heat lamps or the fragile nature of day-old chicks.  They want to get juveniles that are 8+ weeks old.  The value of the bird doubles here.  Instead of going for $3-$10 they goe for $6-$20.  Some people don't want to wait for eggs and will pay $10-$25 for pullets that are ready to start laying
4) Then there are show birds.  You may have 100 Opal Legbars that can lay enough eggs a year to make a backyard flock happy, but if you put all of them in a poultry show There would be a lot of them that would be disqualified and not be judged for placing.  Those that do not show quality fine for back yard flocks but 4H youth and other exhibitors will pay a premium for the show quality ones.  You may have 60-75 out of the 100 that are "show quality" in a good line and only 25-40 that are "show quality in a poor line.  Quality is defined by consistency so the exhibitors are going to pay three times as much for stock from the line that produces 75 show quality birds as they do for stock from the line that only produces 25 show quality birds.  Also if 100 Opal Legbar were to ever be in a big poultry show only 1 of them would win best of variety.  The best of variety bird is going to be worth a lot more than the other 40-75 show quality birds.
So...having said all that if you have Opal Legbars for laying hens then the hens could go for $15-$35 depending on the demand in your area.  If you live right outside a big city with backyard chicken laws that allow hens but not cockerels then the demand for hens (and collectible type rare breeds) will be high and $35 might be too low for their value.  If you live in an area when everyone can have cockerels and most have chickens then even at $15 you may struggle to sell a point-of-lay Opal Legbars.
The Opal Legbar isn't really a show bird yet.  It doesn't have a proposed APA standard of Perfection and while it falls under the umbrella of the Cream Legbar Club it is still just a project variety right now.  Still, it can be shown without a Proposed APA standard and if it wins best of breed, best of variety, or best of class it may be of interest to someone working on the Project Opal Legbars as breeding stock.  Some people will pay $40-$60 for breeding stock.   
So..I would estimate the value of Opal Legbars between $15-$60.  The founder of the breed can probably get $60 for her top breeding stock.  People that do not have well-established breeding lines and are just breeding them for backyard laying flocks and not working toward improvements in the variety/breed will see value vary by the demand in their location.