Good rate.
If you do go to San Miguel de Allende, the best Airport is either BJX (Leon/Guerrero, also known as Bajio or Silao -- the town where the airport is) or Queretaro (QRO) with much more service into the former. Much more air service to BJX than QRO. You can also go to Mexico City, take an express bus (from the airport, or north station; usually first-class or deluxe - with a restroom, movies and maybe a hostess, seriously) to or via Queretaro. 3 hours bus from MEX to San Miguel de Allende. Make a stop at the ATM machine (cajera automatica) first, and get your pesos - you will need them (10 MXN / $1.00).

From Leon or from BJX/Silao, take a Flecha Amarilla (yellow arrow) / Primera Plus bus to Guanajuato. Taxis are expensive, so do as the locals do. Primera Plus is the nicer "first-class level" of buses run by ETN network. F/A and similar do not have restrooms, so know how long your stop is at a station or go-before-you-go. Bus station bathrooms cost 3 pesos (turnstile makes change from larger coins) and get your toilet paper off the big roll at the entrance. "Ladies" usually has a cleaning lady who keeps them very clean - tidies between uses - you can tip if you wish. DO know the names "Silao" and Guanajuato because you may need to change buses there... buses run every 20 minutes. From Guanajuato buses go through Dolores Hildalgo (site of the "grito de Dolores" [cry of "Viva Mexico"] that marked Mexican Independence" and on to San Miguel de Allende ... the whole area is full of Independence history. You may take a Flecha Amarilla (or red "Bajio" bus / Primera Plus bus or another "Herraduras de Plata" (silver horseshoes) bus. Buses every 40 minutes or so out of Guanajuato.
Total trip: 1 hour Leon-BJX/Silao-Guanajuato plus another 1 to 1 1/2 hours to San Miguel de Allende.

**** If you miss Guanajuato, you have missed a gem (Note: The bus station is NOT in the center of town...pay 4 pesos each way to/from town center to see a MARVELOUS city!) Give yourself 3-6 hours and walk the town / take the funicular to the top of the mountain, and see the underground tunneled streets, markets, Cervantes / theatres, and street performers / musicians -- no begging noted. I love this city, and its culture and history -- and this is a temperate climate in the hills (in a canyon, too) that Mexicans dream of. I found interesting things on every corner and bend of the main streets.

From Queretaro (: It is a $25 taxi ride to town / $20 back to the airport (QRO -- service on CO and DL 1x/day only). Know that, since it is rural...not sure of any bus service to it directly. QRO has a Roman-style aqueduct on arches; site of Maximilian's defeat and demise-ending Spanish domination, and lots of history. Also a great business city with a new UNAM university
Once in town, go to the "Central de Autobuses" and take a Flecha Amarilla (Terminal B)/ Primera Plus (Terminal A - twice as much) or Herraduras de Plata. More rural. About 1 1/2 hours from Queretaro bus station to San Miguel de Allende. Buses leave every 40 minutes on F/A... less often on PP.
-- It is then 4 pesos on a bumpy school bus about 1-2 miles into town at San Miguel de Allende, or take a taxi. Tourist maps near San Felipe church ... go to the Cathedral / town square, especially on a Saturday - watch a wedding procession with Mariachis. Some begging noted in the town square. Many artisans and upscale shops. A sleepier town than Guanajuato.

(Much Longer route from Queretaro)-- via the lush valley and fertile plains (Bajio) through Celaya, Salamanca, and Irapuato. Know "Celaya" and "Irapuato" because the buses to Guanajuato go through them...and you may have to change there...
This would take 3 hours to Guanajuato, plus the 1 1/2 hours to San Miguel. I'd do this only if you plan to start quite early and see the town, and then end up in SMA. Take the "first-class" bus if you want a restroom on board, and you will get 1-2 movies as well.
--Note that bus stations do have a "left luggage" office, with claim checks...just be smart.
--25% discount for teachers with ID; Also have student and senior citizen discounts on buses.

Beware of airport departure taxes - Usually $25 if other taxes included in your ticket. $48 if not included in your ticket (such as airline non-revs and bulk tickets). There are ATMs in the ticket lobby.... usually can't split dollars and pesos -- one or the other.