This is one category that is worth watching for on the usual closeout sites, Last Bottle, WTSO, etc.
Lots of better than decent Champagnes come up at around $25. As always, check for ratings and Cellartracker comments before hitting the buy button.
Envoyer offered the Marc Hebrart brut NV and Jose Michel brut rose NV for under $30 recently…ok they were $29.99 but I think they are a good value play.
Most of the sparkling wine I buy is in that price range, but I’m a big fan of pet-nats and other sparkling wines that aren’t Champagne or attempting to be like Champagne. Ones I’ve liked include Texier Rouletabulle, Mosse Moussamoussettes, N.V. Maison Brulees Altérité, Renardat-Fâche Cerdon de Bugey, and Pepiere Les Bulles. Brun’s Cremant de Bourgogne and Foreau’s Vouvray Petillant tend to be quite good as well, and are more along the lines of what many expect from sparkling wines. .
Some of the wines I’ve listed are pink, some are off-dry, and some, like the Mosse, are kind of weird, but each has its place.
I don’t know if you have it in your area, but I do like Pinon’s Vouvray Brut NV - made with chenin blanc so not a champagne substitute, but a very decent sparkler with does gain complexity over the short to medium term. It ought to be towards the lower end of your target price range, perhaps even scraping in below it.
Just a tip: if you want people to know what you are recommending, abbreviations like that do no good. It also prevents anyone from using the search function to find info about a wine later.
For about $25-30 I really like the Gloria Ferrer Royal Cuvee. For less than $20, +1 on the NV Roederer Estate Anderson Valley Brut. The Costco in my neighborhood used to put these on sale for ~$15 around the December holidays, which was a screaming deal. Don’t drink a lot of sparkling wine in our home but always had to pick up a bottle or two when the sale came around.